"He who marches out of step hears another drum." ~ Ken Kesey
https://www.youtube.com/watch?v=YYSfw0H-0NoI do it SIMILAR to the above video.I would push the wrench a bit more than needed so when I tighten the bolt and allow the frame to sit, the other two corners would be the non flat ones.Then with the same corner that I just tighten I would slowly loosen the bolt (not to much) and tap gently (the mesh tension would reverse the rotation) to lower the other two corners. Once flattened I would tighten the bolt.I hope it makes sense.
I've been able to flatten them when I used to stretch the smaller ones manually, but i'm doing these on a roller master.