Before doing anything use common sense
I've been selling decals for many years now and have been asked this question many times. I'll tell you what has worked for me. First you must remove the old decals. I usually use a hair dryer to warm the decals so the adhesive softens. Use common sense here since you can also soften the paint if you keep the heat in one location. I warm the decal then I start at one end lifting the corner as you are pulling up the corner use the hair dryer and focus on that area the decal should lift. On a painted frame If this doesn't work you'll just need to pull the decal off bit by bit and clean off the residue. On a Chrome or Polished AL frame you can use the blow dryer and if that doesn't work lacquer thinner, acetone will dissolve the decals finish and eventually the decal itself will begin to come off as you wipe. It would be wise to wear gloves when doing this. With the bulk of the decals off the residue on a chrome or polished frame can be removed with the same chemicals previously mentioned. On Painted frames I remove the residue with Goo Gone (tar / bug remover) sold at auto parts stores. Windex has worked for me and so has dish washing soap. I spray the residue and work my thumb back and forth the glue will begin to clump up and then can be wiped off. I've used Rubbing alcohol for final cleaning too.
Applying the decals
Surface must be clean residue free. Decals and their adhesives vary so this is a general instruction.
I know placing the decals in the right location seems to be a common problem. The stick it on pull it off method can get expensive especially if your buying my decals. On a pressure sensitive decal you can use this stick it on pull it off method since without the pressure the decal is not sticking fully. On mylar film type decal I wet the surface with a spray bottle put the decal on and maneuver it till it's in the right location. I then work the water and air bubble out by rubbing the surface of the decal.
Thats it hope it helped