Stance Legs should be shoulder width with the legs stagered. Your nose, knee and foot should all be in a veritical line. Keep your elbows all the way in as if you escaping the mount.
Elevation change Before you shoot in it is crucial to lower elevation so you are on the same level to take the legs. If you just try to grab the legs it won't work.
Penetration step The keys in to step between the legs as deep as possible and drive you opponent backwards. From here you can grab both of the legs.
I would suggest either hooking the leg, lifting your opponent, or driving your opponent sideways at a 90 degree angle. But that is another discussion this just how to set it up. Hope this helps.