There's really only two way's it can be done.

Just due to the limitations of anatomy, there's only two ways to do it. You have one guy lie on his back, then the girl mounts him and can either face towards him or away from him. Then the second guy gets on top of them both and squeezes his dick into her pussy. The guy on top is the one that does 90% of the fucking and we've found it works best if the girl is facing the guy with the smaller cock. That way it's less likely to slip out during the action. Use plenty of lube. Longer cocks work better for this.

This is my wife's absolute favorite sex act, (she's Japanese and about 4'11"). I've done it with her probably 100 times or more and I know she's done it without me as well. This will stretch her out. After she'd done this one night with two very well hung guys I put my much smaller cock in her and she couldn't even feel me at all. I paused in fucking her for a couple seconds and she asked, "are you still in me?"