Firstly you need to be able to easily access the area and it must be clean, dry and shaded.
Cleaning involves the removal of everything on the surface – dirt, grease, oil, wax and chemicals. The first three are easily done with soap and water and wax and grease removers (these are also good adhesive removers). The final clean is with isopropyl alcohol which gets the remaining chemicals off. Methylated Spirits does a similar job. Use lint free cloth for the final clean.
This video is quite good, don’t use a blade on air bubbles, use a pin though.
For bow stickers – watch this. Don’t use wet application on a bow sticker, it will come off unless it’s had a day to dry, more if you are heavy handed with detergent.