When you lay out a tarp, cover it in baby oil and/or lube, and have her lay at the other end with her legs spread out. You then make the East-Side gang sign with your hand and slide along the tarp until you insert your first two fingers in her pussy, and then your pinky into her asshole.
Yesterday we tried the East-Side Slip n' Slide but I missed and ended up with road burn on my pecker.