I use a sharp chisel of appropriate width and CAREFULLY scrape away the decal while trying tp keep scratches to the insert to a minimum. Don't use a mallet or anything to propel the chisel - I find that chisels give great control because of their handle length and blade widths.
Follow up with turps or shellite to remove any old glue residue.
My next move is to build up the surface of the cleaned insert with spray can acrylic clear top coat (car parts stores have this) be sure to mask and block sand between coats until you are happy the clear build up is flush with the p/f surface.
Then of course apply the decal using some sort of slipping agent to set it accurately.
Hope this helps.
