I have never used the Windex but I do use lukewarm water with a few drops of liquid dish soap. On a large area like you are talking about use a pump sprayer. If the decal is vinyl with a peel off backing, peel back just a couple inches of the paper and fold it back. With the model sprayed wet, position your decal and 'test' the alignment. If satisfied srat removing the papaer and laying the sticker down and smoothing as you go. If you make a blooper you only have to pull up a little bit because the rest is still attached to the paper. Once you have the entire sticker in place, if you wetted the surface well enough you can still slide the sticker around. When you are satisfied of the positioning start blotting it from the center out with paper towels. If your decal has to go around any compound curves, use your heat gun on low or a hair dryer and warm up the decal and it will fit around the compound curve more easily.

Once it is in place, squeegeed dry, let it stand untouched for at least 24 hours.