Spray the model with glossy varnish. This will make sure the decals will not or hardly been seen after the fitting process.
I would spray the decals first with a clear varnish, so the print doesn’t come off when they are too long in the water. But if you are efficient with applying, that doesn’t have to happen. When the vanish is dry, cut them out to size.
Using Microset and Microsol you get better adhesion. The brand Ammo has Decalfix for Microsol, Microset is just vinegar.
Wet the surface with Microset.
Take the decal to apply, leave it to soak in ordinary water for about 30 seconds and take it out with fine tweezers. Put it in place, then slide it off the paper backing. I use cocktail sticks for it.
When in place, press with a cotton bud to clear the Microset. Just press and don’t move, otherwise you move the decal.
Let dry for 24 hours. The apply Microset to let the decal really follow the texture of the surface.
After 24 hours, spray with satin varnish.