I would attach the solid first, then the pieced, then the solid one at a time. To do it with just one seam at the corners you need a piece of fabric that goes past the corner by the same number of inches as the border is wide. Lets say your main part is 8 inches long, then your 1st border needs to be (8 plus 4 plus 4) 16 inches long. Then you add the points area. If they are 4 inches long from corner to point you will need 12 inches for that 1st border. For the center pieced border the side will be 16 plus 4 plus 4 equals 24 inches, for the point it will be 12 inches plus 4 plus 4 which is 20 inches. Do you get the concept? You will be mitering each corner. Here is a page on youtube that is all about mitered corners. One will probably help.
http://www.youtube.com/results?searc...ac.ySjHgnxue7o