Move The Litter Box
At some point, your cat's litter box is going to have to be next to the toilet so your cat can get used to his new bathroom area. There are two methods of doing this.
Move the litter box a little closer to the toilet every day to make your cat more familiar with his new bathroom situation. Every time you move the litter box, make sure your cat is familiar with the new location to avoid any potential accidents. You can move the box more or less gradually depending on how responsive your cat is to the move, and the distance your toilet of choice is to the litter box. Just keep a close eye on your cat to make sure there aren't any accidents during this transition
This is probably the easiest method. Simply place the litter box next to the toilet and get your cat familiar with the new location. Accidents are more likely with this method, so only use it if your cat responds well to change.
Raise It Up
Raise the litter box up towards the toilet seat a little every day. You can use pieces of cardboard or phone books to accomplish this. Raise up the box more or less depending on how well your cat responds to the change. Every time you raise the box up, take a small amount of litter out. Keep raising the box up until it stands at an equal height with the toilet seat.
An Inch A Day
When the litter box is at the same height with the toilet seat, begin pushing the litter box onto the seat about an inch per day. Keep this up until the box is sitting entirely on the toilet seat. Continue to gradually decrease the amount of litter in the box until there's only a thin layer of litter left.
Toilet Training Device
At this point, you'll need a toilet training device to help your cat along. There are two options here. You can either make one yourself, or buy one. Whatever you choose, try to make sure that the training device is strong enough to support your cat's weight, and use a flushable litter like Better Way Flushable Cat Litter if possible.
Make One Yourself
To make a training device yourself, start by getting a bowl or aluminum pan that will fit inside your toilet bowl. Lift the toilet seat and place the bowl or aluminum pan inside it so the toilet seat holds it in place. Place some litter in the pan, and then get a piece of wax paper. Make sure the paper is big enough to cover the opening in the toilet. Tape the paper over the toilet bowl and shut the toilet seat. Now, you have your own homemade training device. Be sure to clean the litter every time your cat uses it to encourage repeated use.
Buy A Training Device
A training device like the Litter Kwitter Toilet Training System is made specifically for toilet training your cat, and can be placed directly in your toilet bowl. Like the homemade device, you'll have to clean it each time your cat goes.
Transition To Toilet Water
If you made a training device yourself, cut a hole in the wax paper, and make it about one inch larger each day until the paper is almost gone. If you bought a training device, simply remove one ring per day until there are no more rings to remove. Remember to decrease the amount of litter in the pan each day regardless of which method you chose. When the paper or rings are gone, you're free to remove the training device.
Remember to flush the toilet every time your cat uses the bathroom. This promotes better hygiene, and it will encourage your cat to use the toilet more often.
Reward For Good Behavior
Remember to reward your cat every time he successfully uses the toilet. This will help him make positive associations with the activity, and he'll be more inclined to keep using it.
With a little time and patience, your cat will be using the toilet with little effort at all. Keep at it, and have fun!