The jezzail is fired in the shooting phase exactly like a crossbow, bow or other missile weapon. Like a crossbow the jezzail cannot move and shoot in the same turn, so if you move during the movement phase you will be unable to fire in the shooting phase. To shoot the jezzail first turn it to face its target (this doesn't count as moving). To determine if the warpstone shot hits its target roll to hit using the crew's BS as you would for a crossbow, bow, etc. As Skaven have a BS characteristic of 3 this means they will require a 4 to hit a target in the open at up to half range. If you score a hit work out damage as described below. If you miss then the shot sails into the air, strikes the ground, or comes down harmlessly somewhere and has no effect.