Summarize:

Your thumb should be on top of your fingernails. When each thumb is bent at the second joint, they will rest parallel to each other and mimic a pair of lips. Practice lightly kissing the shape of your "thumb lips." Try lightly running it over your thumbs and around the inside edges. Gently push it between your thumbs. Try to find a good pressure that feels right.
Curl your left hand loosely like you are making the letter "O". Tuck your right thumb into the opening on your left hand. Place your mouth gently on your thumbs. Experiment with your tongue. Practice kissing both gently and more forcefully.