If you use the "water slide" decals, like those from Micro-Scale, you need to apply them to a surface with a gloss finish.
Floquil makes a clear gloss finish that comes in their regular paint bottle, & I think Testors makes one that comes in a spray can.
Either one will work, just don't apply them too heavily.
If yopu try to apply decals to a flat finish, they will become "silvered", which means you can see the decal film around the lettering.
You can believe what I say, because I have a knack for learning everything the hard way!
Also,
I know I've said this several times on this forum, but at the risk of repeating myself,
If you're new to this, practice on some old, or junk equipment before you launch an attack on an expensive kit....once again, the voice of experience speaking.