This is the classic seam associated with denim, particularly when it comes to jeans. It’s really deceptively easy to do. (It does help if you can sew pretty straight.)
Here’s how you do:
Start with WRONG sides together. It just feels weird, doesn’t it? ;)
Sew at your normal seam allowance. This is a permanent seam, so backtack at the start and end. Do not clip your starting threads yet.
You’re going to be pressing the seam allowances over the one side…
…then rolling the seam allowances in towards the seam.
I strongly recommend cutting down the inner seam allowance by half, to reduce bulk.
If you are making up a pattern with flat felled seams, this is one of the few times that you’re within your rights to have two pieces in the same seam with different seam allowances – the inner portion of the fell at 1/4″, and the outer at 1/2″. I just find that way too confusing for words when I sew, so I don’t do that. Your mileage may vary.
Once things are trimmed, roll the seam allowances in. If you are inclined towards pressing and pinning, now’s the time. I didn’t – I find that a finger-press will generally do. I specifically avoid pins on felled seams because the fell tends to wibble a little where the pin was. Also, I hate pins.
Take your work to the machine. Line your needle up about 1/16″ from the edge of your roll. Remember those threads you didn’t cut earlier? Hold those with your thread tails – you’ll have a smoother start.
Voila! A seam that is perfectly sealed outside…
…and on the inside.
The classic flat fell is great for mostly-straight seams. Curves are another story entirely. That’s where you’ll need another technique. It has the same finished look on the outside, but inside it’s a whole different beast. Got jeans on? You’re probably wearing an example of the fakey-fell…