Step 1:Sew the fronts together.
Usually, a pattern is marked where the pattern designer recommends that you stop the zipper or end the fly. Because I often take 2 to 3 inches (5-7.5 cm) out of the front crotch curve, I end up with a really short fly. I extend it down an inch. Where ever this point is, I’m calling it the bottom of the zipper.
From the bottom of the zipper down, you want to sew with a regular stitch. From the bottom of the zipper and up, you want to use a basting stitch. Sorry that I forgot to take pictures at that point.

Pants on table, wrong side up. Fold
both fronts to the left, leaving one
extension pointing to the right. Pin
zipper in place, with side lining up
to center front. You will be sewing
the side furthest from center front.

Use a zipper foot to stitch close to the zipper
on the side sticking out from the pants fronts.

Turn zipper over. Fold the extra fabric down
under the zipper. The extra fabric may extend
under center front. Pin and stitch about 1/8
of an inch (1 mm) from zipper. This holds that
extra fabric in place.

Now your zipper is attached to both fly extensions,
but no sewing shows on the front of the pants.

Butt fly template against
the center front seam. If
you don’t have a fly template,
you can copy one from a
pair of pants.

Pin pants front to the fly
extension underneath.
Make sure everything is
nice and flat and smooth.

Using your usual foot, sew fly stitches in place.
I always start at the bottom. If I start at the
top, I tend to twist the fabric.

Overlock the edges of the fly shield. I cut off
some fabric from the bottom to form a curve.
The curve is optional. There’s nothing wrong
with a rectangular fly shield.
If you don’t have a serger, you can overlock
with your sewing machine. Use your overlock
foot and a zigzag stitch.

Fold your pant fronts off to the side, so that
you only sew thru the fly shield and extension.

After you’ve sewn the side of the shield
to the extension, tack down the bottom
on the opposite extension. This is
optional, but it will keep your fly shield
from flopping around.