The best way to not have air bubbles, and it's easy since our units are waterproof, is to put the unit horizontally and make a pool of water on the screen before putting the protector.
And make sure you have pre-cleaned the unit screen thoroughly and dried with a lint-free cloth or paper.
Myself, I use wipes made for cleaning lenses.
It's only a 10 minutes job and works like a charm every time!