Here's another easy way to sew very narrow, accurate inserts.
Sew the first seam of the insert (orange) with a ¼" seam as usual.
Press the seam allowance away from the insert, turn it over and place the raw edge on
top of the next piece of fabric to be added (green). Pin the edges.
Line up the
left edge of your quarter inch foot with the first seam and sew the second seam.
Trim the seam allowance if necessary and press open. The result is a perfectly straight ¼" insert.
This technique can be used for any width by using other foot markings or the arm-like quilting guide that comes with most sewing machines. As long as you follow the previous seam line, your inset strip will be a consistant width, dead on, no extra steps necessary. Your brain will have to flex a bit at first to sew on the left side, but it gets easy very fast.