There are a lot of sub variations on this particular position, as you can situate yourself many different ways to give cunnilingus to a woman who is laying on her back. A great beginner position is to have her laying flat on her back with her legs completely spread. Approach from between her feet and lay on your stomach between her legs with your lips at clit level. Another great one is to have her lay on the edge of the bed with her legs up in the air. Approach from the floor and as you put your head between her legs, let them drape over your shoulders. She can also try putting her feet up in the air, depending on how what is more comfortable.