The central idea is that the transition from litter box to toilet be accomplished in a series of stages. You make a small change and then give your cat time to adjust before you make another small change. If at any time Kitty gives the whole thing up and pees on the rug instead, you're pushing him too far too fast; back up a stage or two and try again, more slowly.
Ready? First start by training yourself ...
The very most important thing to remember is: Lid Up, Seat Down. And if you are accustomed to closing the bathroom door when it's empty, you'll have to break that habit too. Post a note on the back of the door or the lid of the toilet if you think you or any family members might forget.
Begin by moving Kitty’s current litter box from wherever it is to one side of the toilet. Make sure he knows where it is and uses it. Next put something under the litter box to raise it, say, about an inch or two a day. For example use a stack of newspapers, a phone book, a cardboard box, however you don't want the litter box sliding around and making Kitty feel insecure. Tape the litter box down if you need to. Get another box or phone book and raise it a little higher. Continue this process until the bottom of the litter box is level with the top of the toilet seat.
At the beginning of this process, your cat could just step into the box; later he began jumping up into it, until at some point he probably started jumping up onto the toilet seat first and stepping into the box from there. You've been purposley keeping the lid up and the seat down, of course, so by now your cat is thoroughly familiar with trotting around on the open toilet.
Next measure the inside diameter of the top of the bowl at its widest point. Buy a metal mixing bowl of that diameter. Don’t use a plastic bowl as this won’t support the cat's weight and will bend, dropping into the toilet bowl and spilling litter everywhere, not to mention startling hell out of the cat.
Now you move the litter box over so that it's sitting directly over the toilet seat. Take away the platform used under the tray. If your cat is unsure of the change, split this into two stages, moving it halfway onto the seat and then fully over.
Here's the best bit. Take away the litter box entirely. (Ta da!) Place the metal mixing bowl inside the toilet bowl and lower the seat. Fill the bowl with about two inches of litter, flushable litter is easier if you can get it.
Watch your cat using the toilet in the metal bowl. Check the number of feet he gets up on the toilet seat (as opposed to down in the bowl of litter). The higher the number, the luckier you are and the easier your job is going to be ...
...because next you have to teach him proper squatting posture. Catch him beginning to use the toilet as much of the time as possible and show him where his feet are supposed to go. Just lift them right out of the bowl and place them on the seat (front legs in the middle, hind legs on the outside). If he starts out with three or, heaven forbid, all four feet in the bowl, just get the front two feet out first. Praise him all over the place every time he completes the activity in this position.
When he is regularly using the toilet with his front feet out (and some cats naturally start from this position), begin lifting a hind foot out and placing it on the seat outside the front paws. Kitty will probably find this awkward at first and try to replace the foot in the litter. Be persistent. Move that foot four times in a row if you have to, until it stays there. Give lots of praise.
Repeat with the other hind foot, until your cat learns to balance in that squat. (There will actually be two different squats, a low one for urine elimination and a high one for bowel movements.) Once he's getting all four feet regularly on the seat, it's all downhill from here.
Begin reducing the litter in the bowl. Go as fast as he'll feel comfortable with, because as the litter decreases, the odor increases. You'll want to be home at this point so that you can praise him and empty the contents of the bowl immediately after he's finished, to minimize both the smell and the possibility that your cat, in a confused attempt to minimize the smell on his own, tries to cover it up with litter that no longer exists and ends up tracking unpleasantness into the rest of the house.
When you feel Kitty is ready for the next step rinse out the metal bowl, put a little bit of water in the bottom. Increase the water level each time, just as you decreased the litter level. Remember — if at any point Kitty looks unsure go back a step or two and try the thing again more slowly.
Once the water in the mixing bowl is a couple of inches deep and your cat is comfortable with the whole thing, you get to perform the last bit of magic. Take the mixing bowl away, leaving the bare toilet. (Lid Up, Seat Down.)
Ta Da! Your cat is now toilet-trained.