This is super-important. Pin the bias tape to the back of your quilt with the raw edges of the bias tape even with the edges of the quilt. Read that a few more times and think hard about what it says. All the raw edges involved will be stacked up together when you get done. Quilt top on the bottom, batting, quilt back, then the two raw edges of your bias tape, in that order.
When you get to the corner, you do the same fold and turn method that you use for the traditional sew-in-on-the-front method. This gives you a nice mitered corner.