Let's face it, we all have built models that were in NMF and wish it looked better, more "real". Or we have seen a model at a contest that had foil applied and wished we could do that. I'm gonna show you one way to do it. Now this is my way and there are others. I suggest you try different ways and do what feel most comfortable for you

For this How To I am using an Italeri 1/72 Ju-86. The process will be the same for a bigging scale with the possible exception of maybe adding rivets.

First decide on the order in which you want to apply the foil.
For this demo I chose a panel on the wing, but normally I would do the fuselage first and work my way out. Again, do whatever works best for you.
First, cut a piece of foil slightly larger then the panel you want to cover.

Apply the foil to the panel and burnish it down with a "Q-Tip". Don't press too hard or you risk marring the foil.

Once the foil is burnished down, you can remove the excess. Be very careful here, as a small slip will mean reapplying the foil or worse a section already done!
Use a new blade, I used a simple Testors knife for this, but I would normally use a #11 blade.
Press down lightly, because all you want to do it cut the foil and not leave a groove in the plastic. Starting above the upper edge, cut on the outside of the panel line and draw the blade past the lower edge before lifting.

This is what you should end up with. Just smooth down the edges and that panel is done.

Next, I'm gonna do the leading edge next to this panel. The process will work for any of the surrounding panels as well.
Following the same procedure above, apply the foil overlapping the previous panel.

Trim the foil, again applying just enough pressure to cut just the foil.

This is what the finished wing will look like. Ready for polishing!

There ya have it. It takes a bit of practice, but it's not as hard as it looks. Try a simple model for your first time and soon you'll be foiling a 1/48 B-17!