After you tack the corners, like Ken said, seal down the edges all the way around. Do this sort of like spot welding or stitch welding. A tack here and a tack there all the while carefully pulling the covering out to remove as many wrinkles as you can. Once you have gotten tacked all around, take the iron and go smoothly all around the perimeter and firmly seal it down. Once the edges are down you can turn to the gun and shrink the inner field. Once the inner field is shrunk tight and the last wrinkles are gone return to the iron and seal the field to any underlying structure where required (sheeting areas and ribs/capstrips).

Oh, before shrinking the field, it would be good to finish up the wing tips. Don't want the gun shrinking that material until you are ready.

Have you seen Minn's instruction videos