First, let me tell you I'm not a professional trainer. Secondly, I'm better at showing dog training techniques that I've learned than describing them but I'm going to give it a try and hope that I make sense.
Stand parallel to a wall where the wall is to your left. You will want to keep a tight space between you and the wall so that when your dog goes into the finish position, he/she would almost have to squeeze through. The wall will act as a straight edge forcing the body to come in straight and the butt aligned. Hope this helps.
__________________ "What I do is wag my tail and lick your face until you feel better!" Guinness  and Penny:  |