Recruit In Current Fief
Recruiting can be done in any fief where you speak the language-the major language. This means that if your language is F2 you can recruit in fiefs where the language is F1, F3 etc but not G1 (German) E1 (English) etc. There are some linguistic anomalies. Not all of France speaks French. Brittany speaks C3 (Breton) so you cannot recruit there. There is one way you can recruit in a fief you own that speaks a different language than you, but has a Loyalty value of 7 or greater, you can recruit in that fief. Since high loyalty is something you should always aim for in a fief, it is possible to raise troops anywhere if you treat your people right. For some players, recruiting can be a real hassle. There are French speaking lands outside France, mainly along the border with the Holy Roman Empire (HRE) and there are Dutch speaking fiefs in France up north. What's more not all French speak French! There are French nobles who speak Italian in the South East of France (these linguistic anamolies exist to this day in some of these areas). This leads to some odd combinations of where you own property and where you can recruit. If you speak the language (and have the local culture and manners) you can just go and round up a few men. You can always arrange to hire troops from another player, something which is frequently done. Whenever you recruit, you must have 2,000 ducats in your purse for each man you hire. If the fief you are recruiting in is one of your own Ancestral fiefs (the ones your character started the game with), the cost is only 500 ducats per man. It may appear to be more than 500 ducats a man, but that is because the system will recruit in adjacent fiefs if the number of troops you wanted were not available in the fief you are in. In that case, the troops from non-Ancestral cost you 2,000 ducats each. The French and English kings do not get this cheaper rate in their own ancestral fiefs.
Recruitment takes days ( a variable amount, 1 to 5 usually). The number of men you get is a function of the population and your stats (Recruitment ability is a minor attribute if you noticed). You can recruit more than once in a season but after you recruit in one fief, there are no men left to hire, so you must move two fiefs in some direction before trying again (recruit, use 4 days, get 300 men, move 2 fiefs, recruit, get 250 men in another 6 days etc). In general you want to recruit in a high population fief you own. Someone else, even you if you move and come back, can recruit in a fief a second time in a season, but the second time someone recruits there the number of men obtained is lower and the time taken is longer, each time. Paris with 150,000 or so is a great place to recruit for example, as are the fiefs with 40-50,000 pop. One way to figure out where you should recruit is to look for fiefs that have four population icons on the GFE. (You can also look up the pre-plague populations of all the fiefs in this document). An area that is heavily recruited in during a season will run out of troops. But next season there will be more troops available for hire.
Raising an army in the game is pretty much how it was actually done in the old days. You went around and collected whoever was ready and able to go off to war. Up to five percent of the population was trained and equipped (to one degree or another) to fight. Most of these troops were not nobles, but commoners who hired out as soldiers when they weren't farming.
To raise a really large army (several thousand troops), a king or magnate would have other nobles help him with the task. Each noble would bring his contingent to a pre-arranged place and off the entire army would go.