The 1st step would be to allow the cat to identify the new location of his 'litter tray'. I used a 13X10X2.5 aluminum baking tray from the supermarket. It doesn't quite fit, but you can 'mold' it.
If the cat can't find this new tray....... slowly move his old tray closer and closer to the toilet...and then elevate it using telephone books, then put it on the toilet seat...then in the toilet (molded tray).
You want to put a normal amount of litter in the tray........ to begin.
The most important advice is
***Don't progress faster than the cat is ready for***
If the cat seems to complain, or seems aggrevated... allow him time to get use to that step.
Slowly reduce the litter, until there is a small layer.... barely sprinkled on.... (the cat won't like this, and neither will you because it stinks).
At this point... I usually dump the litter (flushable) into the toilet, wash the pan and wipe it dry each time he goes........ pain in the butt I know.... but there is a light at the end of the tunnel.
After he is use to minimal amount of litter..... poke 1 or 2 small holes in the tray... the size of a quarter........... this will allow him to get use to the sound of litter falling into the water when he is pawing.
Allow the 2 holes to be far enough apart so you can widen 1 at a time..... and alternate (If you are skeptical, just make 1 hole in the middle). In 1 week, the hole will be big enough where there is room for litter.... but the hole is big enuf to sweep the business into the toilet.......
the cat will start with all 4 legs in the toilet..... then 3, then rear legs in, front legs out..... then 1 leg in.... then 0. Because there is no room for him.
The final step is a HUGE HOLE with 4 corners ont he tray like this |()|, you will have to put littler on all 4 corners for the cat to see, paw then go.... the hole is HUGE all the 'business' will go straight in the toilet. You are home free!!! You can settle for this and just remove the tray when you need the toilet.....
What I did was I glued litter to the pan so it had texture for him to paw, but I didn't need to keep refilling it.
I left it this way for about a week or 2, then I got a new tray, made a huge hole with no litter..... at first he got pissed, so I put the old tray back... but eventually I used a tray with no litter.
After a while I removed the tray..and kitty was fine with the idea. He would sometimes paw the ground next to the toilet before he goes...and sometimes he'd paw the toilet seat before he goes.
When he is done, he will paw the seat, as if he is pushing litter into the toilet..it's hilarious.
Feel free to PM me or AIM me for advice....... I would love to mentor another successful trainee.