Assuming your cat is box trained, move their box just outside the bathroom, and leave it there a few weeks. Then trim the edges on the box and move it into the bathroom, and leave it there until the cat is used to it. Be sure the box is very clean at all times. Then place it on the toilet seat. Continue gradually trimming the edges of the box. Gradually cut a hole in the center of the box and stop placing cat litter in the box. Wait until the cat gets used to this. Keep trimming the edges of the box until it is just a flat piece of plastic with a hole in the center. Once your cat is used to this, remove the piece of plastic. This may be inconvenient in the training phase, because you will need to remove the box/plastic when you need to use the toilet, but it is worth the effort.