Binding corners that aren't square
Getting Around Not-So-Square Corners
Hint: For mitering odd angle corners
As you are stitching toward the corner, fold the length of the binding over to the left so that the fold rests along the edge of the next edge of the sandwich. Stitch to the corner and stop the “equal distance” as you would binding a square corner. Backstitch to knot and remove from under pressure foot.
Turn the sandwich to line up the next edge.
Fold the binding end straight up along the new/next edge, then bring it down on top of and along the new/next edge, keeping the fold at right angle to the length of binding. You will create a small tuck on the left/folded edge of binding and the right/raw edge of the binding will be exactly around the corner edge of the sandwich.
Starting from the very edge, stitch on to the sandwich, over the tuck and down to the next corner. The remaining corners are done in exactly the same way.