I use a heat gun to warm the sticker up, you can also use a hair dryer as you don't need to get it very hot, just warm a bit up at a time and then peel it off with your fingers.

If you have any glue left over you can pop a bit of petrol onto a cloth and wipe it over the area and this will take the glue off.

Give the boot lid a t-cut and polish and then you are ready to go with your new stickers

Hope this helps