Clean the glass/window thoroughly with soap and water, then an alcohol type cleaner such as IPA


Use some masking tape from he outside to use as guide where you want your sticker to be. 

Un peel the sticker from it s backing and carefully line it up from the inside from one side, the slowly stick it on working from one side to the other to avoid any bubbles.

This is how i do all my stickers and they end up as near perfect as possible.  hope this helps....