Back in the early 1980's I just cut out the decals and put them in water, then slid them onto the model. A decade or more later I started cutting around each number/letter, including the insides of the letter "O"/"P"/"A" etc. The first photo is my P-40 that I built in 1984. The 2nd one was put together a couple decades later and the 3rd photo is both models together.
You will note that parts of the "tiger mouth" decal came off the model. I did not know that you had to clear coat the model, then add the decal, then spray on more clear coat. The kit instructions give you nothing concerning how to treat the decals.
2 additional images. Click to enlarge.