Hello!
Some say that you have to carefully cut all of the film off before applying the decal - I always thought that kinda defeats the purpose, especially in case of inscriptions with small letters...
But if you don't cut the film off, you have to use decal solution to melt it away. Products like Solvaset, MicroSol or Mr Mark Softer do the job here, although not in case of every decal manufacturer - it's good to do an experiment first.
And as precaution against silvering, decals should always be applied to a nice smooth, glossy surface.
Decals that are already silvered - that's always tough. If they aren't overcoated with clear, you can always try to use a decal solution, provided that it works with that specific decal. Sometimes it also works to slash the decal (make cuts through the film without destroying the pattern, the cuts should be invisible), and use some king of very thin clear coat to fill the tiny air bubbles cought under the carrier film this way taking care of the silvering. Sometimes this works, but there's no guarantee! Future sometimes works here. It's always easier to put the decals on correctly, than trying to correct it afterwards, so if you notice silvering, you should stop the decaling and work on your process.
Hope it helps - good luck with your builds and have a nice day
Paweł