Someone on the Facebook group Historical Costume Pattern Review asked how to make McCall's Costume pattern #M7003 more accurate for a time period spanning 1870-1900. Now, this pattern does seem to be cut more like modern pants than the Civil War period trousers I'm used to making but changing any zipper front pants pattern to a button fly isn't really that difficult.
PLEASE read through these before you start your button fly.
First, prep your pattern & make any necessary alterations. I always trace my patterns on to white paper so I have the original to refer back to.
Next, trace your pants front pattern piece, omitting the fly extension. Trace the fly extension by itself. (you can add your seam allowance now or when you cut the pants out). I only added a 1/2" seam allowance (probably because I'm use to drafting my own patterns for trousers), I would HIGHLY recommend adding 1"-1.5" instead. I ended up having to use much smaller buttons than I would normally use.
The fly pattern piece should be this side up for fashion fabrics. (cut two with the right side of the fabric up).
And this side up, for your lining fabrics.
Your pieces should look like this when they're cut out. You should have three lining pieces & two fashion fabric pieces.
Next, pin one lining piece to the right side of the left front trouser piece.
Followed by the remaining two lining pieces. You will pin one lining piece to the straight edge of one of the fashion fabric pieces and one to the curved edge. (see image below)
Next sew all the pieces you just pinned. They should look like the pieces in the following image. When you sew the fly piece to the trouser front only sew to the pin, which is the seam allowance for the crotch itself. Turn the fly lining to the inside and press. Turn the other two pieces right side out and press. (pictured: right front extension, left front hidden placket, left front facing)
Pin the crotch seam from the leg to the bottom of the fly. You can kind of see where I clipped in at the bottom of the seam. You'll want to do that before you sew the crotch seam. Once you've sewn the crotch seam, I usually press it open.
Pin and sew the right front extension to the right front. Press seam towards the trouser front. Topstitch close to the trouser front edge.
Next mark your buttonholes on the left front hidden placket. Sew buttonholes. (don't forget to cut your buttonholes open. It's much easier to do it now.)
Now, you have your hidden button placket and your left trouser front.
Lay the button hole placket on top of the left front lining. Stitch in place.
You now have a button fly! (The yellow headed pins mark where you'll want to tack the button placket to the trousers front.)
NOTE: You will have to adjust the length of your waistband on the right front to accommodate the larger fly extension.