I've trained two.
Very easy. Kittens should be kept in one room of the house for a few days after you first get them as they will be scared of everything for a short while. Keep them in one room. Put the litter tray in a corner of that room with them. (Put some paper under and around it for a day or two). Every now and then, just put the cat in the litter tray and let it scratch around a little and after it does its first "job" in there, thats basically all there is to it. Once it goes in there once, it'll be fine for good after that. If ever you need to move the tray to a different location, just take the cat in after you've moved it and place it in the tray. It will then remember where it is. It's really that simple.
Don't have any pot plants in the room with it when you are teaching it and make sure the litter is changed regularly as if it gets too smelly, the cat will look for somewhere else to go. Clean the poop out as you go and change the rest about once a week or so.
