Continue reading to see how to do this.
Stitch a seam at 3/8", wrong sides together.
Press open the seam.
Press the seam with the seam allowance inside.
Trim the seam allowance to 1/8". I've left part of it untrimmed to show work.
Stitch a seam 1/4" from the pressed edge.
And here's the finished outside of the seam. You can see the inside.