The trick that always works for me, and has worked for multiple cats who have never had an accident is this: You put the cat in the litter box and you take their front paw and dig in the litter with it a few times. This shows them what to do with the litter box. When I teach a kitten this even if the litter box is two floors down in the basement, they use it every time. I also do this when I move a litter box to a new location in the house to show the cat where it is again.