I’ve been working on mastering the Negroni shirt pattern and the flat fell seams were one aspect that I just wasn’t happy with on the first version I made. I also put the cuff buttonholes in the wrong place, so overall it wasn’t exactly my best work.
So of course for the next version I chose the softest, floppiest linen possible: Antwerp Linen in Chambray. It’s gorgeous, but made of 100% linen so there’s not much body to it and an incredibly soft hand. These are great qualities in clothing, but not so much for the construction part. If you choose to go this route, I suggest you invest in some good starch, Best Press, Flatter or something similar.
Because of my choice in fabric, I had to do a little extra with my seams, but first let me show you how to do a basic flat fell seam. These are perfect for the side seams of dress shirts and jeans, but are a good alternative with fabrics that like to fray.
First you will stitch a regular 5/8″ seam. I use my walking foot throughout because it works great and I don’t want to change feet unless I have to. If you prefer, you can use a regular foot for this part. Stitch, then press to set your seam.
To make sure that your seams are as even and lovely as possible, measure your cutting line, 1/4″ from the seam you just sewed. Mark this on the back piece.
Trim along line with sharp scissors.
Head back to the ironing board and press from front, pushing seam toward back. Then press right sides together again, just lightly. Lay the piece down, long side of seam on the bottom and slowly fold over the cut edge so that raw edge *almost* meets the seam line. Be careful not to let it hit or go over the seam line, though. Keep it even and do it slowly. Taking care at this point will make it so much easier later.
From the back press the seam, then turn and press from the outside, being careful that the seam does not unfold. Pin in place from the outside. Use your walking foot at this point and stitch with a heavy 1/4″ seam (between 1/4″ and 3/8″). If you keep a slight pull on the fabric as it feeds in, you’ll be able to keep the seam together better and have less (or no) puckering.
Inside view:
Outside view:
Bam! It’s beautiful!
Now, let me say, I learned this with a bit of trial and error. My first version of the side seams ended up with wobbly seams:
And I failed to get a nice tight edge on the inside. This caused the seam to open out in some place, especially around the armholes. The curves were challenging and they really got fouled up the first time with entire sections coming open and I stitched those armholes a few times, getting more and more frustrated each time. No matter how much I pinned, it just wasn’t looking right. 
So I unpicked all the top stitching on the side seams and armholes. Let’s just say I’ve been known to throw projects away rather than take out stitches, so this was not my first choice. It was the only way, though. I unpicked, press that longer seam allowance flat, then refolded and pressed it to fit closer to the stitching. I’d been lazy the first time.
I pressed it down and made sure that the old stitching marks were visible, but close to the folded edge. 
Then I hand-basted them all down rather than pinning them. This was the key for working with the linen. It kept it all where it was supposed to be and allowed me to ease in the sleeve cap seam allowance much better.
I then stitched it like normal, pulled out the basting stitches and gave it a good press. This is the inside view of the armhole now: 
And the outside view.
Taking the extra time to trim the seam correctly, press carefully, baste the seams and using the walking foot for the top stitching made all the difference in a professional-looking finish.
Now on to the cuffs…
