When I bottle fed orphaned kittens and it was time for the litterbox, I kept them confined to a small space. They are too little to remember where the litterbox is located when they suddenly have to go.
What helps is to keep them confined, use small low sided pans, and after they eat or wake up to set them in the litterbox.
I used Dr Elsey's brand of litter called "Kitten Attract".
For temporary litter pans I used aluminum pie pans or baking pans. You can throw them away after kittens get bigger.
It will be important to get them in a good habit now.
Also, it helps to get rid of any other urine soiled places from the other cats. The pet stores usually have a good urine cleaner. PetSmart here has "Urine Off". Hope that helps.