They usually mount like an old-time sail in a mast. There is a rope sewn into the edge (like piping on a cushion seam), and that edge slides into the awning rail on the side of your camper. To buy the correct size, you need to measure the awning rail on your camper to get the width. The awning rail looks different than the regular rain gutter on the rest of the trailer because of the groove for the awning, so it should be easy to find the ends. As for the depth of the awning to get, that's a matter of choice. Most of the folks here seem to favor 8', front-to-back. You can also get Marti to make an awning for you, she's in the "awnings" section of this forum, and everyone loves her work.
Kristi does a wonderful job on awnings, too, and is very helpful - and is now offering awnings for the front windows which I definitely plan to get! I don't know what her regular website is, but her Facebook page is good. www.facebook.com/vintagetrailerawnings