I agree, bring the plastic inside, let it warm up and then leave them inside for a few days after. I have always thought using windex and floating is harder, because the graphic can float and as your getting the windex out, it has more chance to stick at a little be off from where it should be.
I flip the decal over on its back. Lay a metal stright edge in the middle of the grahic......take an exacto knife and run it along the straight edge lightly to cut through the backing. Slide the stright edge over about a 1/2 inch and cut again. You have to be VERY careful to cut lightly as to not cut into the color on the decal, because it is printed on the back side of the decal. Peel the 1/2 inch strip from the middle. Align and place the graphic in the correct spot. Work the from the middle to the top and then the middle down. Had an extra set of hands help you to peel the back back, as you use your plastic straight to work it slowly to assure you have no air bubbles.. Dont worry about the edges that wrap under or anything. Get the flat portion down then go back and work the small edges, etc.
I have gotten VERY good at it over the years. Had a couple guys tell me I should charge people to do it for them. Just take your time, and do not rush the job.