As soon as babies start to venture out of their box I place a shallow tray in front of the exit so they HAVE to step into it , I use shredded kitchen paper to start with, you could also put a small amount of MUM's litter that she has 'peed' on for them to sniff at, it gives them the idea as to what to do.
My kitten is four weeks old and she uses her liter box I have watched her get in and use it I used to stimulate her over the liter box and tell her to go potty here when she a couple accidents on my couch is when I put the liter box out and Id pick her up after she peed on the couch and put her in the box and she learned to crawl in there and go....
Around 6 weeks you should start showing them where the box is, and the more often you do this throughout the day, the better, they should pick right up on it. Don't spank them for any boo-boos, as this might cause a fear of the box, and their bones are very brittle.
Start putting the kittens in the litter box when they start to walk. I put a book or some thing for the kittens to get on to get in the litter box.
Whenever they start using the restroom around the house