An acid is basically the same as a soul except the lead foot is backside and in the royal position instead of frontside, i.e., your front foot is pointing the opposite direction of a soul grind. Skate up to the rail or curb or whatever at about the same speed you would for a soul grind and come in paralell to it. Jump up onto what ever you are going to grind and pay attention to where your feet will be locking onto the rail or curb. This trick just takes a little bit of concentration and practice.