Believe me, this is a tough technique to pull off.
1. When you are mounted, make sure an elbow is positioned on the mat and inside your opp's thigh.
2. If both of you are wearing a gi, you can grab a fold of your op's thigh. Another variation is make a kimura hold (place free hand on top of the hand with planted elbow. The latter hand should grab the wrist of the other.)
3. Make sure your legs are placed inside your op's legs. Straighten your leg nearest to your planted elbow.
4. This is the tough part. Your body should move diagonally as if rolling your weight to another side. To do this, simul: shrimp out+use kimura hands to push your op's thigh away. Easier said than done.
Another dela Riva variation:
1. The op in your guard raised his leg farthest from you.
2. X-guard his thigh immediately.Make sure you place your opposite foot on top and wrap his thigh with a figure-4.
3. Wrap your hand around his heel and pull it to straighten the leg (his instep should be parallel to your ear).
4. When he attempts to stand, Grab his free foot with your free arm using the form in No. 3 except you don't have to straighten it.
5. Push him down.
6. When he's down, slide your top foot towards you as you lift your other foot towards him.
7. You can grab a collar to make a scarf hold (kesa gatame).