Cats are born pre programmed to use litter for toileting. They love to dig into a loose material and bury their faeces and urine.
If they fail to exhibit this behaviour, programming involves placing the kitten in a large cage or small area with smooth floors eg laundry, bathroom or toilet (close the lid!) and provide a choice of clean litters, eg sand, soil, commercial letter. The kitten will almost certainly use the litter as they dislike eliminating on non-textured surfaces. If they use a drain hole, place the tray over it.
Dirty trays are one of the main causes of trained cats refusing to use a tray. Clean at least daily...