1st off make sure you are working in a VERY clean environment, with NO dust airborne, otherwise you will never get it on right.
Next, start at one end of the phone (I found the top was best) and if the pack came with a squeegee, use that to smooth the protector out as you apply it, a small amount at a time. If it didn't come with a squeegee, then make your own, either out of stiff card, or possibly even a credit card.
Take your time and move down the length of the phone slowly, lifting the protector if you see a bubble, just enough to squeeze it out.
You should be able to do it without bubbles if you take your time.