Back in the 70's I had to apply a variety of decals..........
Varnish : The absolute pits, for older bikes. Apply varnish to the frame or the decals and stick to the frame. Stand back to admire a complete pigs ear.
Water Slide : Slide on and off like Airfix decals. I place the emphasis on off as that's what they did. Not as bad as varnish but close.
Immichrome (Carlton) : Simple stick on, the backs never lined up and the theory of pricking bubbles with a pin is just that......a theory.
531 type transfers / most mid 70's genuine decals : Peel off tissue paper and stick decals on using the backing paper. Rub down and remove backing paper and then pick the bits off the backing paper and try and fill in the missing bits on the frame. soak off the backing paper if time allows. better than all of the above.
Don't remember any vinyl replacement decals for road frames, children,s frames had some though
What bike is it?