I have NEVER been successful sewing both sides with a sewing machine...always looks awful -- and I've sewn since I was 6 (okay too many years to count). I cut 2 1/2", fold in half, sew on the front with machine, fold over, PRESS, then hand sew. Besides the PRESSING making it look neater, I got a hint from a LQS that's helps on the corners. When you're ready to sew the back down, look at the fold on the corner on the front......it will either be folding to the left or the right. Now......on the back side fold it the OPPOSITE way. It makes a much neater crisper corner.
good luck!