A lot of people hesitate to make trousers because they are deterred by the assumed difficulty of sewing a zip on a pair of trousers. That zip is not placed straight on the center front, thus it is not symmetrically positioned. The only difficulty is due to the confusion between right and left side.Our brain is easily puzzled by that.
It is much easier than people think.
Step 1: Preparation
To avoid the confusion between right and left side at patternmaking stage, I draw and cut two identical front : the right and the left both have the fly at the center front.
I will trim one of the fly later at sewing stage.
Step 2: Sew the Crotch
Put the two sides together right side against right side. Stitch the crotch up the beginning of the fly.
Open and iron along the center fronts.
Step 3: Choose Which Side the Zip Will Be Stitched
Now we choose which side the zip will be. Right side (man’s trousers)
Or left side (woman’s trousers side)
I choose the left side. I now trim the fly we don’t need (the left, the one the zip will be on). I trim 1.50 cm from the center front.
Step 4: Preparation to Stitch the Left Side
Draw a line at 0.5 cm from the center front and fold along that line and iron
Step 5: Sew the Left Side
Place the zip on the folded bit, align the edges and pin the zip ready to be stitched.
Put the zip low enough to allow seam allowance at the top ( the waistband will have to be stitched above the zip)
The crease line on the picture below show the center front. The zip has been displaced under the fly. The zip and the stitching line are hidden under the other side when the two center fronts join (when the trousers is closed)
Step 6: Preparation for the Right Side
Align the center front, close the zip and cellotape to baste.
Turn over and cellotape the loose side of the zip along the fly. Be very careful because the left and right need to be perfectly leveled : the two sides of the zip need to match.
Turn over the fly part for easy sewing. Open the zip and sew the zip on the fly.
Step 7: Finishing the Stitching
Now both side of the zip are stitched and they are perfectly leveled.
On the right side view, the left and right are leveled and the zip can close easily.
Step 8: Topstitch the Fly in Place
We can draw the topstitch line for the fly boundary (with air or water eraser pens)
We can stitch the fly on the wrong side following the outline of the fly.
On the right side, I left the drawn line while I stitched on the inside following the outline of the fly.
See the difference
The two methods are good. Stitching on the right side always give you the most accurate line but stitching on the wrong side guaranty a nice curve as we follow the edge of the fly to sew.
The most important is to stop the topstitch line below the metal stopper of the zip.
Step 9: Underfly to Finish
Prepare an “underfly”. the length is identical to the zip length
Place the underfly along the left side of the zip
Stitch the underfly through the left side of the zip
Stitch the underfly to the fly in the bottom corner on the fly seam allowance.