I recently made some canvas liners for wire baskets, and because I didn't want the seam allowances to show through the holes of the baskets, I used the flat felt seam. You know, the seam that is on your jeans. I watched a video of how jeans are made, and as I suspected, someone in a factory, feeds the pant legs into a machine that makes the seam in one swift zzzihhhhh of the special flat felt seam machine. (Editing note: I always thought this seam was called a flat felt seam, and when I searched it on the internet, as such, I found plenty of entries, but after looking it up in the dictionary, and other sewing resources, I now know that it is actually a flat felled seam. )
I also watched a couple of nice tutorials on making a flat felt seam in the comfort of your own home, but honestly, they were a little tedious for me, but that's just me. Impatient I guess. The tediousness involved a lot of pressing and pinning, which is certainly necessary sometimes, but because I make slipcovers and work mostly with heavier fabrics, it's not always practical.
Several years ago, before I started my slipcover business, I worked in an upholstery shop. There was no use of pins there, which was a shock to me at the time, and though they had an iron, as I recall, when I asked to use it one day,a search was called to find it. So, here is my flat felt seam instruction for the impatients of the world. (It works best with fabrics that are a little heavier, that stay where you put them.)
Because I started out making a tube that had only one seam, I positioned the top layer of fabric with 1/4 of an inch allowance, and the bottom about 1/2 of an inch of allowance.
After the seam is stitched, the larger allowance is folded over the smaller. If you are making something from a pattern and it isn't practical to move the top allowance over before stitching, then sew your seam, and trim the top allowance.
Next, fold the seam flat, with the "folded over" allowance on top, (this is where the pressing and pinning would come in, if you wish to add those steps) and stitch close to the edge.
Here is the finished seam.