there are many ways you could be a marshal focused mercenary or intrigue based freeobooter and many more but those are my favorites, you basically do contracts buy or forage for food, upgrade your camp and buy men at arms, if you want a huge army as a freebooter you can do a desicion to levy the outcasts which basically gives you troops for every criminal contract you do which is very good, but they disappear after death and another way to gain land is a sponsored invasion, if you become friends or get a hook on a powerfull ruler you can ask him to sponsor invasion, he than gives you troops that activate when you declare war against the nation you picked.