Finishing Arm Seams

Thought I would show this because many shirt patterns don't as they tend to keep things simple and this can be a little complicated.  When finishing arm seams, paws down, the best finish is to roll the edges and sew them down.  This is how most off the rack dress shirts are finished, you may notice.  This second row of stitches reinforces the arm seam, dividing which stitching will take the stress of various arm movement, instead of placing it all on a single seam.  It also looks clean and neat, and lets you add your own flair, such as different colored top stitching.
It's really simple to do this finish, but it requires some pattern editing to do.  The sleeve opening on the front, back, and yoke (if used) does not have to be changed.  It all has to do with the sleeve itself.  To the top of the arm sleeve, add roughly 1 cm to the seam allowance.  By the top, I mean the part that will be sewed to the arm hole.  I say roughly 1 cm, you can use more or less because this is really a design choice.  The more fabric you add, the wider it's going to be, and the wider your top stitching will be.  See the following pic, this is how much extra fabric I have on the sleeve compared to the arm hole.

When attaching the sleeve, it will now be longer than the edge of the arm sleeve.  After sewing the sleeves on as normal (right side to right side, etc.), open it up so the pieces are laying flat, like pictured.

Now take this excess fabric and fold it so the edge is touching the seam you just sewed.

Now fold it over again in the same direction, while using your thumb to pull the sleeve fabric out.

This will enclose both raw edges, as shown in the picture.

This is the wrong sides of the pieces, with the excess fabric of the sleeve folded over.
Start folding at the yoke and work your way out.  Well, you don't have to, but I strongly suggest doing it this way, because the yoke is where the fabric has the most curve and is the most difficult spot to do this folding over at.  From there, work your way out to both edges.  Just fold a little bit, pin, and repeat.  I like to pull the sleeve away from the body of the shirt before pinning, to make sure that I won't sew over the shoulder, which will give it a weird looking seam when worn.
When sewing, follow the edge of the arm seam, not the edge of the fold.  This way, you will ensure the top stitch will be parallel.  I prefer to sew when viewing the right side of the fabric, to make sure I won't have any visible puckers or folds, as shown.

When following the fold, you'll get a straight and even stitch that looks perfect when done:

Now just complete the shirt as normal!
With this folding over method, it doesn't have to be that precise when it comes to how much fabric is on the sleeve.  You can have a bit excess, unless your fabric is very sheer.  Then, you'll have to take extra care in making sure the fold is exactly the same all the way around, or it will look very strange when worn.  Easy to achieve by, after sewing the sleeve to the body, cutting the excess fabric off the sleeve until it's even all the way around.   
-Cat

This totally radical and bodacious cotton print was bought at Robert Textile, located in the Los Angeles Textile District at 510 East 9th Street.  One of the best shops in the district for finding cotton prints, with amazing prices and a friendly owner.  I saw this fabric and was instantly drawn to it.  It's like the most 80s fabric I've ever seen.

The black gingham fabric is from Michael Levine.  No links for this one because it is a horrible fabric that I do not recommend for clothing.  Very loose weave, it's like working with stretch fabric.