The Letter "A"
To form the letter "A," you put your hand in the fist position, ensuring your thumb is outside of your fist, upright.
The Letter "B"
To form the letter "B," you take your four fingers (excluding your thumb), and place them together, and act as if there glued.
The Letter "C"
To form the letter "C," you take your five fingers and form the letter "C," just as you normally would.
The Letter "D"
To form the letter "D," you take your all of your fingers (excluding your pointer finger) and form a circle. While your four fingers are in the shape of a circle, you straighten your pointer finger. If your hand is in the correct position, it should look like the lower case version of "D."