Hi,
different materials offer different ways to do this!
If you have only printed the decals on paper and cut them out, then use spray adhesive and spray the back of the respective decal with it.
If the decals were printed on self-adhesive paper or printable self-adhesive foil, it's easy-peasy, because you just need to cut them out and stick them on. I always have my decals printed on foil in the advertising studio. The quality and the price fit there!
And you can check out this article:
https://www.flitetest.com/articles/skins-make-your-planes-look-ridiculously-good
Please note that this is replacing the entire paper layer. This is not the case with my small decals!