Mom should take care of most of it, with just a little help from you.
What I did was when they were nearing 4 weeks of age, I put a smaller litterbox just for them a couple of feet from their nesting box (Mom still had hers in the bathroom). You'll find they'll play in it, kick it all over, everything but. lol But they're getting used to it.
If you see them in the litterbox playing, it can't hurt to go over and either take one of their paws and show them how to dig, or I would siometimes just dig with my finger once or twice while they were looking.
Between Mom and that, they'll get it sometime ususally lbetween 4 and 5 weeks.
Jeja Vu did quite an interesting thing with her litter. Her box is fairly large (another reason why I wanted a smaller, lower one for the babies.)
Usually fastidiious, I noticed suddenly she started kicking litter out of her box all over the place. I finally discovered why.
She was teaching the babies to use the litter, but didn't make the connection with the 2nd litterbox. Since she knew they would have trouble getting overthe box, she kicked out some litter, made a pile and was teaching them to use that. I was so proud. That was pretty smart! LOL
I cleaned it up, and they kept using their litterbox and Mom's and didn't miss a beat. They were able to use Mom's also.