(I hate all clings, never get those straight)
Borrow a level from your SFT and make a masking tape line that is level, then bend back a 1" horizontal strip from the top of the cling, line up the top of the cling to the tape and adhere with squeegee, continue to peel back an inch at a time while moving the squeegee from the center to the left and right.
Doing it with this method I was always able to get them on straight and without bubbles.