To fire a bolt thrower, select a target according to the normal rules for shooting. With the target chosen, pivot to face the target like any war machine, and roll to hit using the crew's Ballistic Skill, just as you would for a normal missile weapon. All the normal To Hit penalties (except firing Multiple Shots) apply to a shot from a bolt thrower – it is essentially just a massive crossbow, after all.
If the shot misses, the bolt hits the ground or sails into the air, only to land harmlessly some distance away from the target. If the shot hits, damage can then be resolved using the rules given below.