The best way is with double sided tape. Not the fat foam stuff, the thin adhesive type. Make sure the old decal is completely clean (all old residue removed). Then take it and stick it onto the double sided tape. If the sticker is wider than the tape you may need to do a stripe or two extra. Trim around the decal before peeling the back ing off. Pull backing off and stick!
A few things to keep in mind.
1) make sure you use the very thin stuff.
2) some tapes have a yellowish tint to the glue so this is best used on solid coloured decals.
3) if you are using clear backed decals you will need to experiment with tapes to ensure you find one with a clear adhesive.
