Kittens learn from their mother and by the time they move to a new home they can usually use a tray. If a kitten is missing, start not with training but by checking three things.
The tray
It should be big enough: about one and a half cat-lengths from nose to tail base. A cat outgrows a small tray, and the misses start exactly when it no longer physically fits. For a kitten the sides must be low, or it simply cannot climb in.
The location
Quiet, away from thoroughfares, not next to the food bowl and not next to the washing machine. A cat will not go where it was frightened, even once. If the flat has several floors, or there are several cats, you need more trays than cats — at least one more.
The litter
Kittens often prefer fine clumping litter: it is softer underfoot. Strong fragrances repel — a cat's nose is many times more sensitive than ours. If you change litter, do it gradually, mixing the new into the old.
What to do about a miss
Clean the spot with something that breaks down the odour, not with bleach: to a cat, bleach smells rather like a marker and invites a repeat. Do not rub its nose in it and do not tell it off — that works with no animal and only adds fear.
Put a tray where the kitten went. After a week or two you can move it gradually, ten centimetres a day, towards where you want it.
When it is not about the tray
If an adult cat that has used a tray for years starts missing, that is usually not behaviour. Cystitis and urinary stones produce exactly this picture, and there is nothing to wait out here: it needs an examination. Especially if the cat sits in the tray for a long time, cries, or there is blood in the urine — that is urgent.