I was congested last night, so early in the morning I came downstairs and fell asleep in the recliner. I woke up and checked what was on and saw a sewing program on PBS, so I switched over to it. When I tuned in they were showing a technique for making mitered corners for place mats. Later I went down to the injector to give it a shot on some scraps.
First you make your rolled hem on both sides of the corner one side at a time. In the show they pressed the fabric. I don't use an iron on SIL, and the creases go away quickly, so for these pictures I made the fold and ran a sharpie down the edge so they would show up. After creasing each side in turn, lay flat and trim as shown here.
Next roll the newly cut edge. This photo shows the first fold. It is critical that the next fold is at the intersection shown here.
Now go back and roll each side. I normally don't pin much, but I had to for this to stay. Then you just run your stitching along the hem and make the turn. Finished corner from both sides.
I think if I use this technique in my winter tarp I'm making I will sew the mitered corner hem first, then place my reinforcement and roll normally.