In the men's shirt making course I took in the Fall of 2012 at FIT, I learned a flat fell seam treatment I had never heard of before which I will detail below. The sleeve for a man's shirt is installed flat, not in the round. So once the sleeve cap is attached to the garment, you must sew the garment side seam and sleeve seam. You start sewing this seam from the bottom/hem part of the shirt up all the way through to the sleeves. The trick is how to sew a flat fell seam in a sleeve which will now be closed. Seam allowances are 1/2 inch.
1. At the underarm area, fold up toward the right side 1/4 inch and place 3 pins. Press in between the pins. Remove the pins. This is just to give you a little more control and precision in this area. For the rest of the shirt edge, you can just fold under at the sewing machine 1/4 inch using a 1/4 inch foot as a guide as you edgestitch along the fold. IMPORTANT: You will do this only for the back, not the front.
2. Pin the back to the front right sides together abutting the edge of the front side of the shirt to the raw edge of the back side of the shirt where you had folded up the 1/4 inch seam:
3. Sew together 1/4 inch away from where the raw edges meet. It is easy to do with a 1/4 inch presser foot:
4. Press enclosing the raw edges. This is the wrong side of the garment:
5. Turn to the right side. Press on the right side as well to make sure the seam is nice and flat. You sew from the right side because it is important that the outside topstitching be perfectly straight since it will be visible. Start sewing from the bottom/hem part of the garment with the edge of the presser foot 1/8 inch away from the seam line. In effect, you will be sewing 3/8 inch away from the seam line:
6. Here's the tricky part. You must continue doing this all the way into the sleeve which you had closed and made into a tubular shape in Step 3!! The way you do this is you bunch up the fabric in your hands and then sew 1 to 2 inches at a time. As you sew, you will get more of a nest shape around the foot as the machine is pushing the sleeve fabric to the back. I think of it as a nest on which I must sew only on the bottom flat part:
Here is what the wrong side of the seam looks like. You can see that 3 rows of stitching are visible:
Below is the right side of the sleeve which, after all, is what counts. It does look pretty good!
Sewing books and pattern instructions have you trim the inside seam to 1/8 inch for a flat fell seam. The beauty with the method described in this post is that there is no trimming involved. The seam is stronger and more durable because you have a full 1/4 inch seam inside. On the other hand, the topstitching on the outside is 3/8 wide and you may prefer the 1/4 inch topstitching often seen in jackets, jeans and many other garments which you obtain by using the more widely known method described in the sewing books. Also, the more common flat fell seam produces a seam that can be used on either side.
However, there is no right or wrong way. There is frequently more than one way to do things in sewing and I believe that knowing more than one way of doing something is good to have in your sewing arsenal. When you sew your own garments, you have the freedom to choose which technique you like best for the particular garment you want to create.