" Can someone briefly outline the steps for proper decal application on plastic surfaces?"
Looks like you gotten great info on all the basics so i'll add a few variations. For decal application and setting the basic Micro scale instructions work well. The Microset/microsol combo will do most jobs. If you need a strong setting solution for places where rivets or other bumps are causing the decals to bridge over them then Solvaset is the stronger solution, just be careful if using Solvaset for the initial wet application as it can soften up wet decals enough to spoil them.
If the car has a smooth painted surface the basic application method works well. If the car is rough or unpainted plastic then it's possible to decal over a thin wet layer of Future floor finish, the Future will seal under the decal and eliminate the white sheen that a rough paint surface imparts, and will also make decals stick well to unpainted slippery plastic so colored plastic cars can be decaled without painting them. To decal with Future just trim the decal and then paint on a very thin layer of Future in the spot where the decal will go then apply the decal to the wet Future and use the brush or a tissue to squeegee the excess future from under the decal. You don't want the decal to float on a thick layer, just sit tight against the car on a thin layer. After a few minutes the decal might wrinkle up a tiny bit but it will flatten back down as it totally dries.After it's dry a spray coat of Dull coat will blend the shiny future into the car body color. Future works well on smoother cars but since it doesn't soften the decals it's not as good over rivets or ribs.
Some general techniques I've found to make it easier is trim the decals as close as possible with a sharp exacto knife or fine pointed finger nail type scissors(the scissors are probably easier to use). Dip the cut out decal in water then place on a saucer in a small puddle of water till it slides off easily, don't force it till it's free all over it's surface or it might tear. Some decals require more time in the water and maybe a bit of finger massage on their back paper surface to loosen them up. Once it slides off the paper easily I slide it part way off then grip it with some fine pointed tweezers and place it in the thin Micro set layer (or Future layer) then use the tweezers points to slide it into the final alignment. Eyeball the decal carefully to make sure it's level and square with the car body or any nearby rivet lines or weld seams, and that any lettering has the proper spacing . Once it's where I want it I blot with tissue to get any excess solution off it then wait. Don't try to re-position a softened decal as it will tear, get it where you want it as soon as possible( if there's a lot of decals on a car side just wet and apply a couple at a time then a couple more after the first ones are in final position, etc.).
Decaling takes some practice but is a great skill to develop as it allows one to customize their cars as needed.......DaveB