Should be in the FAQ.

If not, then it's get an army led by a general of at least 8 units. Then go to the tab where you can hire mercs and all that jazz. In the lower left corner of that tab, you get two buttons. One of which has a cross on it. Click that and ta da! Crusader army.

Be sure to hire the crusader mercs as they are dirt cheap (110 for an armored sarge equivalent, 220 for a heavy knight, and something like 70? for a good flanking unit).