Few positions offer such deep penetration.
Simply "fold" your partner's leg back and place it on your shoulder - see picture.
Rock gently at first before building up to greater gusto - no one wants to do a hammy mid-root.
She will need to be well-wet for this position, too, so use your free hand to stroke the boobs.
And make sure you switch legs to ensure supple quadriceps all round.
ILLUSTRATIONS BY: JETHRO QUIZON