This issue was covered within the last year or so in this forum, so is deja vu all over again; but that may be just the way it is in this medium - just takes some getting used to.
First a dry iron is set just hot enough to make a good folded crease that will last a few minutes without damaging the material. Careful experimentation is required on scraps to get the right setting on the iron.
Next, the edges of the fabric panels must be marked with a line where the fold is to be. The width of the thin strip to be folded over depends on how wide you want the felled seam to be.
Next, the material is folded on the line, and just the edge of the fold is pressed with the edge of the iron. The working surface underneath must be something that will not melt or ignite. I use a ping-pong table made of a wood composite we used to call homasote. Next, the two panels are joined with the folds overlapping as they would on the finished lap felled seam.
Pins are inserted to hold the seam together, and are placed perpendicular to the seam, and between where the two stitch lines will be. With practice, the material can be laid flat on the table, and the pins inserted using the fingernail on one hand as a thimble to press against the fabric and pinpoint in order to get the pin to come out of the fabric at the desired point.
Next the two stitch lines are sewn. Each line penetrates four layers of fabric.
Stitching the first line, I pull the pins out just before the work enters the presser foot. Usually I can sew the second line without having to re-pin, because the fold in the fabric has been pressed by the iron to stay in place long enough to finish the seam. I sew very slowly, to insure the material is just where it should be when it passes under the needle. This requires a good machine motor and variable resistor in the pedal to operate at slow speeds.
The result is a true flat or lap felled seam that has no needle or pin holes outside of the stitch lines. There are the pin holes between the stitch lines that go through four layers of fabric. After seam-sealing, they present no problem.
The seams on this silnylon fly for a Hubba were sewn in this fashion:
With a "faux" felled seam, as was already pointed out in earlier posts, there is a stitch line where holes in one layer of fabric are exposed:
The stress on these holes is likely to defeat the sealer and allow rain to enter.
Wish I could sew like a pro on a pro machine with a seam folding device. The chances of that happening are between zero and none; so resort is had to the above method. For purposes of measuring pattern panel sizes, it is a little tricky at first getting used to the idea that the fold lines are offset to each side of the center of the seam, but it gets easier with practice.
It seems worth the effort to get a much more durable and water resistant seam.