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First of all, I iron the seam open all the way around the crotch, to flatten it. Then I iron it over to one side on the outside. I fold the seam over to the side that has the topstitching on the fly front - it’s the right side in this case. I just checked the front of the purchased pair of pants I am currently wearing and on them, the fly top stitching is on the left! So you never know. I iron the fell as much as I can on the back of the pants where the seam is straight. When it gets to curve too much, I simply fold the seam allowance over and tuck it in with my fingers, putting pins in perpendicularly as I go. I put the pins in this way because it is easier to hold the fold down and I can get a lot of pins in and I can sew right over them if need be.
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In the next photo, I am sewing the fell on the wrong side (the inside of the pants) using a zipper foot to get close to the folded edge. If you are going to sew over the pins (instead of taking them out as you go), go slowly or you risk hitting the pin with the needle, which can put a burr on it (making it useless) or breaking it altogether. I am at the part where the yokes meet the pants-back and there will be a substantial lump of folded fabric to sew over. Using your hand, turn the fly wheel on your machine and “walk” the presser foot and needle over the lump if it doesn’t want to go by itself. I am also using my right hand (which is holding the camera so you can’t tell) to gently pull the fabric away from the sewing line, so that it doesn’t get bunched up under the line of stitching.
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After this seam, I pinned the side seams and sewed them both. At this point, before sewing the side seams, you can pin the pants on and see if you need to adjust the width of the side seam allowances. I have made these pants before and I haven’t lost any weight recently so I know I don’t need to adjust anything (famous last words). I also sewed the facing on the skirt while I was at it.