I just did this on a quilt that I'm making and didn't know how to do it either. When I was coming to a corner I stopped about 2" from the corner, raised the border piece and sewed the flange down with a 1/8" seam. Then I started it on the next edge the same way. After doing that I put the border piece on top of it and sewed a 1/4" seam, cut it off at the end, pressed the border open then started the across the next side. It worked for me although I don't know if that's the correct way of doing it. Except for the corners I sewed the flange and the border on at the same time.