i always place my kittens in the litter tray straight after each time they eat. i sometimes even help them scrat in the tray with their paw so they get a feel for what it is. Eventually after this happening everytime they eat or if they pee outside of the tray they get the nack of it. They do like to be clean.
I clean out the tray with special spray from the petshop designed to get rid of odours.
You can actually buy a spray that encourages kittens to pee in the box too.

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