Cats are born with the instinct to scratch the ground. The way we taught our kittens was to place them on the litter, and physically scratching the litter with their paws. Trust me, its in their instincts, after a couple of days of you doing that, you will be surprised to find your kitten has done it on its own soon enough!
Teaching a cat how to tinkle and poo in the litter can take 3 to 5 days. :)