Definitely, welcome to figging.

I did end up trying this once after getting into it with my ex... Personally, I'm of the group where it had very little effect. There was some tingling, but for the most part it was rather boring. Worth noting I tried this out of incredible curiousness and not the desire to try it before doing it.

She, however, reacted rather "pleasantly" to it.

I did things a little differently. Rather than tying my sub up I would either simply make her sit and watch, or take part in the preparation. Despite enjoying it, she wasn't ever exactly eager to start a figging session... I think having the freedom to get out of it before it started made the anticipation stronger, and imposed a sort of mental-bondage/submissiveness. Fi would often say the worst part was the sitting still and not-squirming during prep.

Also, a few other useful positions/ideas:

In pretty much any position, having the subject's legs together tends to increase pressure on the fig. I've been surprised by how many people who do anal play in general, that don't realize this.

Doggy style is pretty canonical for this. Supported or unsupported; unsupported gives your sub the opportunity to try and shy away from anything happening to them, squeezing down on the finger with already mentioned results.

Face down, flat, works pretty well for this also. This has the advantage that you can periodically squeeze your subs cheeks together to increase the effect.

Standing. It's my opinion that figging a subject works best if they're standing. There are so many things you can do standing that promote clenching that it's like an open ended array of ideas. Even simply standing up straight, properly, causes the buttocks to clench slightly... standing on your toes it's a natural reaction to tighten those muscles. You can also do anything that promotes the sub pushing their hips forward while standing, because you -have- to use said muscles for this; you can use something like a dresser (or lateral slat, if you have something like a horse that would work well and give you better access to the genitals if desired), the farther back his/her toes are from where the object they have to press against the tighter they have to clench. A wall also works for that, if you care to have your sub standing facing a wall -- however, a wall also works the other way. If you have them stand back against it, standing up straight the shoulders, butt, and heals should all touch the wall, if the finger protrudes enough to touch the wall that will help (so long as it's not in danger of completely entering). Protip: If your heels and shoulders are both against the wall, it's practically impossible not to use your ass to keep your hips off of it.

Also, standing with your feet angled outwards will increase pressure.

Hopefully that gives you some more ideas of how to have fun with this.