Here is a guide I found on applying decals and stickers, from Footman James, of all sources.
http://www.footmanjames.co.uk/blog/decal-application?utm_source=newsletter&utm_medium=email&utm_source=SilverpopMailing&utm_medium=email&utm_campaign=FJ%20Newsletter%20-%20February%20(PL)%20(4)&utm_content=
This might be helpful.