I don' t want to get too technical, but I work as a surgeon and I think I have a few easy tips. Once you have completely opened the abdominal cavity, your cut should end at the pelvis. You can then take your fingers and gently push the bladder off of the inside of the pelvis. After you have cut the trachea and esophogus up high in the chest you should peel the entire contents of the chest and abdomen down toward the pelvis. It is important to make sure that you have the kidneys peeled off of the muscle. The kidneys are connected to the bladder by the ureters. Once you have the bladder freed up along with the rest of the chest/abdomen contents, you should be able to then cut around the anus without puncturing the bladder. I always do this with one hand inside the pelvis pulling traction on the rectum (this makes cutting easier) and keep the blade of my knife pointed toward the bony wall of the pelvis. Good luck, its not nearly as hard as you might think.