Get one of those hammer stapler guns that the guys use for installing tar paper for sliding or roofing and it'll go so much faster. All you do is just whack away kinda like a hammer and bam it's in. It so much easier also to do by yourself because you should always be building your room by yourself. Home Depot sell them for around $30 and if you can't afford it keep the receipt and take it back when you're done. They'll take anything back. If you bought thin/cheap plastic use some HVAC silver duct tape before you throw a staple in it and that stop tearing, but if you bought some nice thick stuff and use a lot of staples you'll have no problems. I also roll all my edges on the bottom and top so I'm stapling through multiple layers of plastic which really helps keep it in place.
Cheers