"Set" goes on the model before the decal. Think a place to set it. "Sol" goes on the decal after it is in place. Best summary I've seen recently (and within this forum), from rjkplasticmod, although it doesn't mention using a q tip and water to wash off any residue after the decal dries:
I always put the Microset on the model, with a small amount of water mixed in with a brush, where the decal is going. After the decal is applied, wick the excess Microset & water away with a paper towel. At this point, I usually gently press down on the decal with a folded pad of paper towel to make sure the decal is seated and to help remove any air that may have been trapped under the decal. Work from the center to outer edge of the decal, but do it gently. I then apply more Microset to the top of the decal and then DON'T TOUCH IT . Wait 20-30 minutes and inspect the decal. If it has snuggled down OK, you don't need the Microsol at all. If the decal needs more help to conform to contours or panel lines, then put a small amount of Micrsol on the decal, but again don't touch it. Some wrinkling of the decal may now occur, but don't panic, walk away and leave it alone for an hour or so. When the decal has dried, the wrinkling should disappear and the decal will be snuggled down. Never, Never try to press down the decal when it wrinkles after decal solutions are applied. Some solutions like Solvaset are even more aggressive than Microsol and may cause some decals to self destruct. Lots of modelers use it with success, but for modern decals, especially from aftermarket suppliers, I prefer to use the Microset & only use Microsol when needed to get the decal to conform. Hope this helps.