Leave some kitten food out where they can reach it, but don't force them to not drink mommys milk. Mommy-cat will help wean them by laying on her tummy so they cant drink. In time they will only eat the kitten food an leave mommys tummy alone.
Just to get them started, I give them canned food. Then I soak dry kitten food and add canned food. They will walk all over the food at first, but they'll soon learn! Mommy will love to get to it, so watch out!