So here goes.
Sew your seam with the normal seam allowance, with the wrong sides together. It feels wrong. Do it anyway.
2: Trim one side of your seam, but leave the other one long
3. Press your seam toward the long side
4. Fold the long side over the short side, so your cut edge lines up with the seam you sewed and press it down
and it looks like this
5. Fold this bit over and press it down, so your raw edge is all snugged in there and can't get out.
See? It looks great! But you still have to sew it down.
6. Sew it down, getting as close to the folded edge as you're comfortable with
Done! It looks professional, and it wears well. I do this for the center seams in pants. It takes a little longer, but most of it is done with the iron so you don't get little folds and bunches once it's sewed down. There's probably an easier way to do it, but this is the way that works for me.