Your tarp or
awning material is sewn around a "rope" on one side. That is then slid into a C channel ment for awnings. The other end is held up by poles inserted through grommets. Much like the old tent doors of days gone bye.
No pattern needed other than knowing how big you want your
awning to be.
Here is a good site for ideas:
Marti's Vintage Trailer Awnings