Learning can happen so quickly that the casual observer may be unaware that any active instruction has taken place. The kitten should be placed in the litter box upon waking, after meals and vigorous play.
Most kittens take over and successfully use the box.
The trick is to keep watch to make sure that the box is the only spot the kitten uses. Imprinted on that texture, recently homed feral and stray cats may have to be actively trained to use a box filled with clay litter. While some strays catch on quickly, others don't. If the cat refuses to use the box at any stage, back up to the last stage at which he was successful. Sexually mature cats use urine and feces to mark territory and advertise for a mate.