Great comments on yesterday's post, at least some of which speak to the subject of today's post - what to do once you've got your underpants gnome plan in place. It's all well and good for me to say "Come up with a plan, then fill in the gaps that present themselves" but it might be a little unfair to not provide at least a little guidance on how to do so, and what you can do once you've got the trick working.
First, one of the easiest and most powerful tricks you can do is run through the list of your characters and ask yourself "Where does this plan intersect with this character?". Does it threaten someone or something they value? Does it use something they want? Is it taking place in their favorite restaurant? Would it just REALLY annoy them? Or perhaps does it have an element, such as an end, they might be inclined to support? If you don't have a good answer for one character, that's ok. If you don't have a good answer for any of your characters, then perhaps you need to consider the plan.
Second: The underpants gnomes need not be villains. Underpants planning can apply equally well to heroic or even indifferent outcomes. The characters may even find themselves as the agents responsible for delivering someone else's UG plan, which can get very interesting if they don't have the whole picture. One of the most classic twists is to have the player's handle step 2, not realizing that step 3 is something horrible.
As an aside, because it's a classic, it's kind of overdone and ham-fisted. If you must do a twist, have step 3 be something reasonably value neutral (like getting the bad guy a resource or removing an obstacle) but which will then be used in the unstated step 4. Also, if you do this, plan for your players figuring it out, and see if you can give them the tools to screw the guy who's trying to screw them. Few payoffs are as satisfying.
Third, though related to the second: The steps need not be uniformly bad or good. As Joe pointed out in the comments, having a REALLY ADMIRABLE step 3 paired with an UTTERLY ABHORRENT step 2 can make for a powerful mix. Similarly, a benign step 2 with a bad step 3 can be a great play driver. Not just for the twist scenario, as above, but even when played straight by an NPC willing to say "Yes, this bad thing will come of it, but compare that to all the good you'll do!". Fun stuff!
Fourth, and this one definitely got tipped in the comments, the true secret of the Underpants Gnomes is that you really only need to be concrete about step 1 and 3. When someone has a premise and a goal, things can go wrong in the middle, but they can regroup and keep trying to pursue the goal. As a GM, this means that so long as you keep your eye on step 3, you can be flexible about the shape that step 2 takes, possibly even requiring multiple attempts at step 2. Goals make much better planning aids than processes in this regard.
Fifth and last - once you have the trick of it, start juggling. Underpants Gnome Plans are surprisingly easy to maintain once you have them in play, so start introducing a few more. Where one such plan can blossom into a decently fleshed out arc, several of them can turn into the kind of tapestry that keeps a world feeling alive and in motion while giving the GM a bottomless bucket of resources to draw on to keep things moving.