Use a snow glove of thick or thin material that fits your child's hand snugly. The glove will make up the base of your baseball mitt.
Study images of baseball mitts or borrow a glove from a friend to learn the shape and pattern. Search for "baseball mitt patterns" on the internet, or be creative and sketch out your own.
Measure and cut strips of foam rubber to wrap about the fingers of the glove. Sew the strips together to form a finger-length tube and sew the top shut. Repeat the process for each finger and the thumb.
Measure and cut strips of leather or plastic fabric to fit snugly around each of the fingers covered in rubber. Sew the strips around each rubber tube, and sew the top shut. Sew the tubes to the fingers of the glove.
Cut out more strips of leather, about the length of the fingers, for the webbing between the thumb and fingers. Sew the strips into place.
Measure the palm and back of the snow glove and cut out sheets of foam rubber to size. This padding will protect the palm of the hand. Sew the back and palm piece to the fingers, one at a time, leaving the bottom of the fingers open so that your child can slip the glove onto his hand.