Mine use the toilet to pee and poop!

There are many methods, but this is the one I used.

Get their litterpan moved to the bathroom. then make it a smaller, round pan that can fit in the toilet. After a day or two, place it ONTOP of the closed toilet. after a day or two, drop it IN the toilet with the lid taped up, so it wont fall on them. As the days pass, pull more and more litter out of it till you only have a dusting on the bottom. The next day DONT wash out the pan it will still smell like their litter pan , and put about an inch of water in it. As they step IN the water to use the pan, they pick up their feet, and place them on the edge of the seat, and will drop their heinie over the edge of the seat to pee or plop over the water. after a day or two, pull out the pan and they will continue to use the toilet.

Make sure you find a way to keep that lid UP! if it falls on them, they will be afraid of that toilet. Use tape, or a huge rubber band. The bumping and knocking against it will cause it to fall on them.

Praise! loads of praise! give them privacy while they figure it out, and be prepared for accidents. They can sometimes drop thier butts over the wrong edge.

Make sure to scrub clean the old area where the old litter box set, to remove any temptations of smells that used to live their. a nice vinegar rinse will help.

Teaching them to flush is another matter. At least there is no more litter. ALL cats have to use it. If you have the litter box anywhere in the house, they will prefer that over something new. One of mine kept trying to go to the tub to pee, until I blocked it off for a day or two.