what I find is the easiest method of applying a film screen protector and worked for me for 2 years whilst working in a vodafone store is the following:
1) ensure the screen is clean of dust, a good cloth in a relatively clean environment does the job, if not do it near a boiling kettle.
2) before peeling off the first layer line the screen protector up along the top of the handset, where you would like it begin. (for the htc one, it would be just below the top speaker grill),
3) peel back the first layer only slightly, line and up press down adhesive side with the top of the handset, easiest done with the top of the handset facing away from you. (benefit of this is, you've pulled the whole thing off and it's less likely to catch dust if only a small part of the protector is visible to wilderness.
4) with the top of the protector adhered to your screen, swivel the phone around so the top is closest to you without letting the protector move from it's position,
5) from here, it is a simple task of taking a piece of cardboard or hard paper and gently push forward from the top, whilst slowing peeling back the bottom plastic until you're at the bottom.
6) you may have some small bubbles right on the bottom edge of your protector, just use your thumb or the paper to gently squeeze them out.
Works for me 90% of the time. Also depends somewhat on the quality of the protector. But I would recommend a Glas.Tr or MIME type protector.