Sewing Tip: When sewing a welt seam, fancy term for topstitching on one side of seam, trim the seam allowance layer located directly underneath the topstitching to half its width. Doing so will minimize bulk providing a smooth, flat application.
After the seam is stitches but before the seam is ironed with the seam allowance flat, trim the seam allowance located on the side that will receive the top stitching down to half its width as shown.
Once trimmed, apply an over edge stitch like serving or zigzag (or any other finish of your choice) to the longer untrimmed seam allowance edge. Iron the seam allowance layers flat pointing in the direction of the top stitched side- the idea here is that all seam allowance layers will plane aligned on the side of the seam that will receive the top stitching with the trimmed layer hiding underneath the clean finished longer edge.
One trimmed, clean finished and ironed, the seam can now be top stitched, catching all seam allowance layers underneath. You will find trimming the hidden seam allowance layer will not only minimize unnecessary thickness along the seam, it will also make the stitching process more comfortable and smooth. The final top stitching will conveniently seal the trimmed seam allowance layer behind the finished, untrimmed one preventing it from fraying and providing a professional seam finish.