I mark where I want the finished edge to be
Then hem – fold the fabric over twice and sew a row of stitches
Then sew the fabric to the zipper such that the finished edge is as close to the zipper teeth as possible. You can even overlap the zipper teeth slightly before it will snag when zipping. Being as close to the zipper teeth as possible will provide more protection from rain.
I have a zipper foot that has the plate on just one side. You can flip it so it’s either the right side, or the left. So foot doesn’t ride up on top of the zipper teeth.
Then, flip it over and do another row of stitches on the back side along the edge of the zipper tape.
I think a zipper foot that automatically folds the fabric would be good if you were professional and had to do a lot of zippers quickly. Same thing with a double needle machine.
When I do a (faux) flat felled seam, for the second row of stitches, I pull apart the seam and press down the flap to try to get a nice flat seam with both rows of stitching taking the load.
I have never waterproofed my seams on mids and never noticed any leakage. The seams angle down steeply so the water naturally flows down.
When I attach something like a guyline to a tent, I have had that leak. I just ignore it, then if I notice it leaking, I seal it at home the next time.