Just baste the sides as you go. If it will fits with your design, I would also SITD between the center and the border. when you are ready to turn, just pin the sandwich to your uptake leader and thread the rest between the belly bar and backing roller. I have found that just letting the weight of the quilt float at this point provides enough tension on the sandwich to quilt the first border. Quilt that, then roll the sandwich onto the takeup roller and pin the trailing end to the backing roller when you get there. Adjust the tension and quilt the last border.