Definition of marshal: "to bring together and order in an appropriate or effective way" but also "lead solicitously".
While anyone can hire mercenaries to act as forward observers and determine direction of approach of the enemes most potent individuals or guard a village borders for a time discouraging bandits or perhaps at higher levels to search the realm for signs of a missing relic or any number tasks suited for violent men in significant numbers often simply opposing the efforts of similar violent adversaries. Further such troops may be engaged in a strike with an explicit goal, such as disabling locations of import (bridges or portals come to mind), assassinating lesser leaders (careful now this might be too much for them), or limiting the delivery of supplies to enemy troops. Such troops must be acquired in appropriate places and contexts (and may not be available in the numbers desired) however in "civilized realms" soldiers are generally available and you can indeed bring them along into risky "adventurous scenarios", In the game this latter may take some heroic convincing, but the mercenaries way of life is dangerous when things are going well so this might be less extreme than it might seem.
In real wars having as many as 1 in 4 soldiers not returning to their families is generally seen as a particularly vicious war. And that is unlikely except where technological leaps make military tactics and strategy unpredictable.
There is an assumption that retreat and fall back, or even surrender and taking prisoners will be common (the latter are often lumped with casualties when evaluating a conflict even though they may make it home.) In effect the condition of unpredictability mentioned results in leaders unable to gauge properly when to fall back when to call of surrender and even how to position troops.
That said under those conditions where you do have unpredictability a slaughter can happen with 85 percent dead, captured or missing.
These rules enumerate what this means in terms of the Heros story and makes it transcend what is described above.
The possessor of this art goes well beyond what can simply be purchased, or even what can normally be accomplished by a typical nobleman with gold and followers. The real difference is for you it isnt about failing or not (It is unlikely significant numbers of your Thanes will be lost or that they will fail in their goal, unless the over all situation goes seriously south (in game terms the skill challenge fails),Yet whenever you use them in a challenging way it temporarily disables/fatigues them (unless you make a good skill check / or a reduced level use). Between ingenious allocation and direction of forces to maximize success minimize their loss and inspire their loyalty earning glory for both them and yourself. The troops so marshaled may be called Loyalists or Thanes or Elites or similar.
In some sense this art is about learning the strategy and tactics and timing but also presentation which allow one to inspire those you guide to do those things mentioned much more reliably and with less risk, this is a Warlord specific practice as it relates to that classes core functions just more broadly applied (note feats such as Martial Prodigy allows you to learn practices reguardless of having the pre-requisites and certain backgrounds/character themes may also enable access).
This art generally includes the responsibilities of maintaining oaths, reputation and/or a family name or more generally a connection to your organization which is the nominal source of your troops within your home "domain" . Particularly for those who have regular access to troops by use of the followers feat or by class ability which allows free performance of the art. Although it is also possible to acquire followers in more dynamic ways wherever there are people for you to inspire.
In pure abstracted form this allows you to efficiently expend resources to bring troops to bear (to gather and guide them) against a problem situation (aka skill challenge).
Generally the effect is your forces help address the challenge and gain an auto success in the challenge after making a skill check which modifies the cost and represents the quality of your application of the forces to the task and combined with the impact of other skill checks in the challenge their survival. As usual a DM may allow a particularly appropriate use to be even better. Troops are particularly good at managing large dispersed areas for instance such as when you need to reduce the collatoral damage of enemies.
In terms of power some of the effects achievable are well within level 1 rituals just of a different flavor and skill base others seem like they are whatever level the Warlord is. That said what level an art can be learned is often a function of flavor more than power in that regards the Monster Manual human Nobles are around level 5 and the guardsman level 3 we could easily decide the flavor of Marshal Troops is appropriate to be level 5. The art even if learnable at level 1 (in support of young King Arthurs and Alexander the Greats and Arslan wannabes this seems like it will be fine) may end up with a learning cost like a level 6 due to its high versatility.
Marshaling troops is a highly versatile art because the things troops might accomplish can be very broad so its base cost may be fairly significant 150 could be appropriate
Before applying troops to a task you gather them (this can actually be accomplished many ways and may use Diplomacy/Religion or even Insight when for instance it can be done by having a hero train the locals) the troops can be visualized as specialists or a mix of generalists etc but regardless they have a role based on the tasks they are marshalled for. The personal guard of heroes tend to be more versatile in some ways but more often scouts than other style soldier. Troops which have been gathered without being applied to a challenging task are sometimes referred to as fresh troops (the first challenge they are subsequentlly Mashaled for is treated as basically pre-paid). Some warlords are less adept at history insist that inspiring/refreshing such troups is the only way to go
Marshal Troops uses the Variable Cost Chart based on History (but sometimes Diplomacy or Religion or Insight to Gather / Inspire / Refresh ) and can be made in karma or gold, and at a level appropriate to the Task Set
Initial Gathering of Troops or Refreshing Troops may often be based on Diplomacy or Religion but Application to the troops is generally History
Sometimes the benefit of troops is significantly reduced from that of a full at level challenge and usually so is the threat/hazard level for the troops. Such as in the following preventing a possible surprise round for the enemy.
Night Watch or Forward Observer: Most of the time you won't have unwelcome visitors when you do however it becomes a potential issue what level of challenge is it?, the primary effect is simply preventing surprise situation - ie with significant pay being called for only when the task becomes hazardous. By limiting engagement with the enemy to observation and reporting the Warlord can keeps risk/costs down to minimums. Used in this fashion by with that level of engagement is approximately the same cost as using the alarm ritual - ie level 1.
However sending troops out to assure the party gain a surprise round on enemies (who might other wise have had surprise on you) is very much an at level effect and can be done only at similar level to the Preys instinct: Double Back.
Building a Camp and Foraging These functions are comparitively less dangerous even than the above AND troops can be expected to aquisition for themselves up to a point - note to self there needs to be real benefit.
The price is not actually about paying daily wages for troops that is really very shortly merely a tuppence for the most part and in a medieval world 5 silver (or perhaps 25 silver) a week even for a skilled soldier. It is more on the order of paying for healing and hazard pay they can be restored via celebratorial activities which also representedthe cost - also the Oaths of Fealty bind you to pay the cost associated with the ocassional death of your Thanes and to expend resources towards healing their injuries and so on. This cost is generally paid after the deployment/towards the goal which is generally the opposite of buying mercenaries. Mercenary cost is also assumed to include looting (which need not occur for loyalists).
Part of the value of troops marshalled is they are not necessarily located in story context exactly where you are they become a flanking approach to the problem. IE they may be a valuable distraction or larger scale intimidation which averts a portion of the issue or removes an obstacle interfering with a heroes action in the challenge increasing their results.
And following a significant challenge at minimum the particular troops will need to rest and recuperate this will often be at least an over night thing but varies by tasks.
Though it has been said that it is rare for this artist to lose significant numbers of troops their following will likely have small attritions and gains (and likely increasing in numbers and or levels as they gain levels). A poor roll on ones Marshalling skill roll especially combined with a failure in the over all challenge can be a very poignant opportunity to explore the relationship of the Warlord to their troops. It is an opportunity to describe a heroic death for some not normally seen as heroes.
In game terms even under such circumstances the troops action contribute an auto success to the challenge. Conversely a good roll might allow reuse of the same troops towards subsequent challenges that day, or mayhaps towards another goal within the challenge. Arguably the maximum expenditure in a day for a set of gathered troops might be based on 1/10 the karma point cost
The following are primary flavor text for visualization purposes when troops are set to basic tasks to reduce the abstraction. But also they do not have to be Playing out actual battles with more than a few minions are likely to be tedious however if one wants more direct mechanized tactical battlefield use of companion troops swarms can enable a number of options which might be more satisfactory for some players and groups.
Example tasks in consideration for within a heroic battle scene include acting as reserves (including Volley from the Rear), Rushing from the Flank, and Defensive Retreat
The above is a big simplification and the larger the groups the more variations are possible for instance a patrol might be
The above are the result of a single application of the art ... how many troops total can be brought to bear? Well sometimes none because ultimately its related to the story but in theory at best one might be able to marshal as many as you have karma and gold to retroactively pay for and functions for their use.
Most Heroic Warlords and Nobles tend to "under use" a personal guard or keep a squad of scouts, and instead they risk themselves while using guardian types as hearty scouts rather than personal defenders and more often call on the help of other heroics who form a inner circle.
Some may call them into play later in the fight so they are not worn down when the action gets heaviest (use them as a second wave actually lets them collect glory)or simply use them in very limited ways
Instead of controlling the battle purely by your genius you taking active part in the tasks you set for your troops thus assuring the soldiery are less injured and taxed while reaping glory and situational hazard pay (looting the enemy forces), ie generally without karma point or significant gold piece cost to you ie you take the brunt of any failures upon yourself in one fashion or another - perhaps your injury (aka healing surge loss or merely the effort spent) saving the life of your soldiers inspired all the rest ensuring success and survival in the challenge.
Idea: perhaps only a bravura warlord build has this option.
This is a way to use your troops "on the fly" is represented by a Combination of two practices Marshal Troops and application of Trained Preparedness (Increase the difficulty of the skill check by 5 with failures sometimes meaning the troops didnt arrive in time and similar issues).