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:
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.
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…