For the bath, get a large jar with a mouth easily big enough for the kitten. Fill with warm water and mix of flea killer. Grab kitten by scruff of neck and dunk away.
If it has lots of fleas you'll actually be able to see them make a run for the dry face. Use a damp rag to get the head.
Here's an example of me doing our cat when she was a kitten:
But if it has them and has been in the house for a few days you easily could already have a problem. The other cat should get a bath too.