Move the cat’s litter box so it is right next to the toilet, keep it like this for about a day.
Then gradually raise the litter box up by placing phone books (or something similar) under it. Repeat daily until the box is of equal height to the toilet. Whenever you raise the box take a little bit of the litter out of the box. You may need to secure the litter box to the phone books or whatever it’s sitting on so that it doesn’t move when the cat jumps onto it.
Move the box over 1 inch onto the seat. Repeat daily until the box is directly over the seat. Continue gradually decreasing the amount of litter in the box until there is only a thin layer (less than 1 inch) of it left.4Replace the litter box with a “training box”. You have a few different options here, but the important thing to remember is to make sure the “training box” can hold your cat’s weight if they step or jump onto it:
Lift the toilet seat and tape a piece of wax paper over the hole so that the toilet looks like a drum. Lower the seat over the wax paper. Add flushable litter.
Lift the toilet seat and tape a bowl or aluminum pan to the edges. Put the seat down so that it holds the bowl in place. (See video below.) Add flushable litter.
Use a commercial training device, sold specifically for this purpose. via
Do It And How 