Years ago, I learned this technique from Margaret Islander videos. This was included in both the Shirts, etc! and one of the Galaxy of Sewing Techniques videos, which I watched in VHS back in the day. They are so fantastic, I might ask Santa to buy the full set of DVDs for me.
Keep in mind that I forgot the specifics of measurements from the cut edge, but I do remember the technique quite well.
I sewed reference lines 5/8" from each cut edge.
|Pink lines 5/8" from each cut edge.|
If you fold 1/4" over before stitching, then it's hard to see if you have butted the top layer right up against the fold line. However, then you can sew a scant 1/4" so you catch all three layers (white+green+white).
Try both techniques and pick what you prefer. There is no reason why you can't sew one line through two layers, press the fold, and then sew right next to that line through three layers. These lines of stitching won't show on the outside.
|Lay one layer 1/4" inside other edge, then stitch 1/4" from second edge (green).|
Stitch right next to the fold of the white fabric, catching all layers. I stitched from the wrong side, with green thread in the needle and navy thread in the bobbin.
On the wrong side, you see both the first row of stitches (navy bobbin thread) and the second row of stitches (green needle thread).
|Both rows of stitching from the WS.|
|Second row of stitching from the RS.|
If you want a centered seam, then keep the seam allowances even between the two pieces you are joining. Make samples because the spacing will depend on the thickness of your fabric (TOC) and how fat you want the seam. For jeans, I would try 3/8" as a starter and adjust from there.
If you want the seam allowance to lay to one side (on a sleeve cap), then trim one seam allowance 1/4" narrower and add it to the other side.
For instance, you may want to trim a shirt body SA to 3/8" and give the sleeve cap a 7/8" SA. Stitch with a 3/8" fold/SA and you will end up with a FF seam pressed against the shirt body. You'll find this easier to do if you gather the sleeve cap with a running stitch and a hand needle first.