There are plenty of tutorials to be found on how to insert a fly front zipper. Some of them are quite cryptic, and all of them seem to give you a slightly different technique.
At the risk of confusing you even further, I'm giving you my version. The tutorial is from one of our shorts patterns, but can be used with any shorts or trouser pattern with zipper at the front. The pattern should include a piece for the fly shield, if not you can cut your own one. Length is slightly longer than the zipper, 2 3/8 " = 6 cm is a good width. Fold in half lengthwise and finish the raw edges.
Using widest possible stitch, machine baste front seam all the way up to the waist. Finish the edges of fly facings. Clip open the seam allowance where indicated by the arrow and press the seam open.
Edgestitch along right side of the fly. Place the zipper right side down, lining up the right edge of the zipper with the centre front seam.
Move all layers underneath to the right side, and on the LEFT side stitch close to the zipper with a zipper foot, attaching the zipper to fly facing only. Then pull the zipper over to the other fly facing as far as it goes - do NOT line up with the edge though. Pin in place. Move all layers to the left side and stitch, again attaching the zipper to facing only.
Using a seam-ripper open the wide-stitched centre front seam. Open zipper and press from right side. On the right side use chalk or baste to mark a guide line and then top stitch along that line, catching the fly facing underneath. Place fly shield on top of zipper on the wrong side, zig zagged edge lined up with the zig zagged edge of fly facing. Stitch in place, along the left side and bottom, attaching it to fly facing only.
Et voilà! Now all you have to do is attach the waistband.
P.S. If it still doesn't make sense to you watch this video by Threads Magazine.
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