how to call a crusade by priest or like that

[Myth]

For a Crusade you have to have at least a moderately good relationship with His Holiness the Pope. When you go to the religion tab you can call the crusade by choosing the target city.

With an Imam, he has to be at least 4 (or was it 5? I can't remember now) piety (the "command stars" of priests). You select the imam and click the little button on his unit card.

If the Pope rejects your offer, try throwing some gold at him and try again. Also, he prioritizes different cities differently. Jerusalem and Cordoba are easy enough, while Vilnus is harder.

[TeHpWNaG3]

When you call for a crusade (or join an existing one) there is a very cool "exploit" that I just can't help doing. This works for Jihad as well.

First, figure out if you can actually get to the crusade location before another faction. Playing as England, I was always being beaten to the city by a closer faction. Most times that faction will camp next to the city indefinitely or lose the battle and then camp there, so know this before proceeding with this strat. The city must be taken for this to work!

Here is the exploit:

Just before the city is to be taken, pull all of your generals out of the cities and add as many units as you can from surrounding areas. Try to add the best units you have (and you can also plan for this ahead of time). Join each stack to the crusade and then immediately take the city. When the city is taken (by you or someone else), the pope will give all of your stacks experience, you'll rise greatly in favor of the pope, and a good deal of money will be rewarded, iirc. The timing of this is important. If you allow another faction to take the city and they stall or fail, your armies will desert or you will be forced to move your armies toward the crusade target, pulling all protection from those areas. If you have a general with not enough units to join the crusade, add mercs or pull in support units from nearby provinces.

A more "noble" or non-exploitive tip for a straight up crusade would be to leave the siege machines behind and bring 3-5 spies instead. The loss in movement points can be devastating to your army through desertion as the journey persists. Plus, they won't take up any room in your stack, and they help protect from assassins. win/win/win While you are at it, bring a whole host of priests, merchants, and whatever have you to the city. Antioch will become "New London" in no time.

ETA:
Tip #2: I find that going to the crusade target by boat (a small stack of boats) is the fastest and safest form of travel. Sometimes by land an enemy army will block your passage. You have two choices, fight or go around, but know that you can lose the fight or significantly weaken your army, and going around will stall forward progress and incite desertion. Try to launch in an area that maintains forward progress at all times (i.e. circumnavigating Spain is a no-no; start from the Mediterranean sea).

Tip #3: Crusades can be a huge geopolitical event. Every faction that joins the crusade is instantly at war with the target faction, and sometimes with the factions they will have to pass through. It will wreck alliances very quickly. Also keep in mind that your reputation with the pope will suffer greatly if you do not join. Controlling the College of Cardinals will be difficult if you snub the pope's call.