FWIW, decals should be placed in warm water for not more than 30 seconds. Then they should be set aside to soak up the water for 5 more minutes. Next you can put a little water on the frame and begin to slide the decal off the paper, just enough to begin to stick it to the frame. Once it's lined up, hold the end of the decal to the frame and begin pulling the paper out from under it, pressing out bubbles etc. as you go. Use a rubber squeegee to smooth it. For stubborn bubbles, puncture the decal a little with an exacto knife to let the air out.
On very old or very thin decals, you can add lacquer which is what they are made of. Spray on a couple of light coats, letting them dry. Then spray on a full flow coat. Let that dry completely. Now you have a problem of lacquer over the edges and the decal won't slide off so you have to trim the very edges of the paper. I use scissors. This is really important with vintage 50 year old decals, otherwise they will shatter when you try to apply them. The new lacquer will solve that.