You can also use vinyl from a sign shop (ask for scraps) or from places like Harbor Sales. It's the same stuff as monokote trim sheets, a sticky-backed plastic film. Apply using windex or simular to allow you to move it around and keep it smooth.

I've done plastic-over-plastic iron on, it's a pain. As was said, work from the center out if possible. Also, really turn the iron way way down. Some of the bubbles are from gass released by the glue as it heats up. Less heat, less gas, less bubbles.