How do I get kittens litter trained?

The mom cat put litter outside of the box even tho i made a little box out of a piece of cardboard that the kittens can get in and out by themselves. They use that and I also found that they have been going to the bathroom on my blanket that I have for them. I need to know how to litter train them before i give them new homes. There are three kittens plus one kitten that I am keeping. I want to start finding homes in about one more week, which they will be eight weeks old.