I've done this before. You'll want to use your zipper foot for this. Sew the zipper in place and when you reach the corner, stop with the needle down. Raise your presser foot, pivot the fabric so you're ready to stitch down the next side. When your fabric is in place, grab the zipper and pull it toward you, pulling so it "turns the corner". You should have a 'hump' form next to the needle. Lower your presser foot on top of this 'hump' and continue sewing.
I hope the directions are helpful!
Christine
http://quiltdasher.blogspot.com/