Fine shirt making involves tiny stitches of 16-22 stitches per inch, so set your machine to a stitch length of approximately 1.5mm before you begin to sew. Stitch out some samples using a fine thread in both the upper and bobbin threading. Measure the results with a ruler to see if you can achieve 16-22 stitches per inch. Small stitches prevent crooked top-stitching and edge-stitching, and also make it easier to turn a corner very precisely and not a stitch or half stitch beyond it.
Narrow flat-felled seams are most desirable for shirts as they create a nice smooth finish on the inside of the garment, and they are strong since they are double-stitched. The finished width should be a scant 1/4". Wider flat-felled seams are reserved for use on heavier fabrics such as denim or corduroy, but can also be found on the armsyce seam of many traditional shirts with a flat sleeve cap. Wider flat felled seams are easier to construct when dealing with the thickness of the fabric at the yoke and the opposing curved edges of the sleeve and armsyce.
Follow these easy instructions to make a perfect flat-felled seam. Always try out a sample first. Keep in mind that one side of the fabric will show two rows of stitching and the other will have one row of stitching that appears to be topstitching. Either side can be used on the outside of the garment depending on the look you prefer.
Sew the seam with a 5/8" seam allowance.
Press the seam open.
Trim one seam allowance to a scant 1/4". Trim 1/8" off the second seam allowance.
Fold the longer seam allowance over the shorter seam allowance. Press the seam towards one side.
Edge stitch close to the folded edge with an edge stitching foot. The opposite side will have one row of topstitching showing.
The armsyce seam can be flat-felled if the sleeve cap is fairly flat. If the sleeve cap is shaped and higher it is too difficult to flat-fell a sharp curve. A shaped sleeve cap can be finished with serging and a narrow row of edge stitching or a french seam. To flat-fell a flat armsyce curve increase the seam allowance on the sleeve cap to 3/4". Trim the shirt body seam allowance to 1/2". Fold the sleeve seam allowance over the shirt seam allowance, and press towards the body of the shirt. Baste by hand to hold the folded edge in place. Edge-stitch close to the fold.