I have been sewing the binding on the front, then trimming. My first trim is to leave about half an inch or so extra of back and/or batt. After the binding is sewn onto the front, using the edge of the top as the measuring line for the raw edge of the binding strip, I then trim the seam allowance to 3/8 inch. But I measure from my line of stitching. Now the binding needs to be pressed open very well. With a 2 1/2 binding strip to start with, that leaves just the right amount to turn under. With straight grain binding, I can do most of it without pinning or gluing. Only the corners need special care. I pin that in place a little before I get there. Once in a while my corners are perfect !