MOM:
Make the sign for "Mother" by placing the thumb of your right hand against your chin.  Your hand should be open.

Note: there are several variations of the sign "MOM."  
An interesting method I've seen is to place the tip of the thumb of your right "A" hand on your chin then open the "A" hand into a "five" hand.
Some people wiggle the fingers while signing MOM.
Some people use a double tap on the chin.

 DAD:
Make the sign for "Father" by placing the thumb of your right hand against your forehead. 
Your hand should be open, and you may choose to wiggle your fingers slightly.

GRANDMA:
Two ways to do this.  One is just to use a large arching motion.  The other is  to use two small arcs.  Grandpa is similar except that it is done starting at the forehead.