I have been working with twill tape as my seam binding on my billboard vinyl wallets. I have used several different methods with varying degrees of success.
This is my best method so far: (1) Fold the twill tape down the centre and steam iron. (2) Figure out how much tape you will need for the seam, plus a length for folding the ends over. (3) Use low-tack painter's tape to hold the seam binding in place. This is especially useful for turning a corner, because you can shape and tape at the same time. (4) Sew in place using a home-made seam guide comprised of a picture-hanger hook, hot-glued to a fridge magnet. It stays in place nicely on the metal base of my Singer Sewing machine. Painter's tape can be used to hold the guide in place. (5) remove the painter's tape.