Begin by fusing the Fly Stand and Fly Facing pieces. Make sure that the Fly Facing is cut and fused right side up.

First you’ll need your fused Fly Stand. Press this piece in half lengthways and bag out the bottom of the square with a 1cm stitch. To do this, fold it in half RIGHT SIDES TOGETHER and stitch a 1cm line all the way across the bottom.

Mitre the corner, turn out and press flat.

Next you’ll need your zipper foot attached to your sewing machine. Lay your right leg the RIGHT SIDE UP on the sewing machine.

Lay your zip RIGHT SIDE DOWN with the edge of the tape lined against the step seam allowance.

Your zip stopper should be lined just above the bottom of the little step. Stitch your zip down using the zipper foot to get up close to the zip teeth.

Lay your fly stand on top of the line you’ve just stitched with the raw edges flush against each other.

Stitch this down at 1cm, stopping right where the lower notch is.

To prepare your binding press it into place by folding the outer edges in to meet in the middle. Then cut roughly into thirds and save two pieces for later.

Lay the right hand leg in front of you RIGHT SIDE DOWN and find your spare piece of binding.

Lay the raw edge of the binding against the fly guard seam with the RIGHT SIDE of the binding against the WRONG SIDE of the trouser.

Make sure there is plenty of binding at the top and bottom, then pin into place and stitch down at 0.5cm.

Next, wrap the binding over to the front to enclose the raw edges.

Start by turning the leg over so that it’s sitting RIGHT SIDE UP and make sure the binding is sitting out and flat on the table. Using small scissors you can then remove excess bulk from the corners of the binding and fly guard.

Then turn up the bottom end of the binding and then fold the rest of the binding over the raw edges to enclose the whole area. Take your time arranging the layers so you get a neat squared off section at the bottom.

Edge stitch into place and then trim the excess binding at the top to sit flush with top of the leg.

Snip in about 0.5cm at the bottom of the seam step, just under where the zip stops.

Flip this little free step out to lay flat then add a 1mm edge stitch down the side of the zip, stopping just before the step.

Lay the fly facing in front of you RIGHT SIDE DOWN. Lay binding on top of the fly stand with the RIGHT SIDE of the binding against the WRONG SIDE of the facing.

Leave plenty of binding hanging off either end. Stitch down around the curve at about 0.7cm which should be in the grove of the fold created during pressing the binding.

Next roll the rest of the binding over to the RIGHT SIDE of the fly facing. Wrap the binding around the raw edge and pin into place.

Then edge stitch the binding down - take this nice and slow, making sure the binding is nice and parallel with no tucks or twisting.

You can then trim off the ends of the binding to sit flush with the ends.

You’ll need your front left leg in front of you, RIGHT SIDE UP. Now lay the fly facing against this raw edge, RIGHT SIDES TOGETHER.

Stitch down this front edge at 1cm stopping exactly at the notch. Do a good backstitch here.

Snip into the seam allowance by 1cm at the notch to where the stitch line is and lay the facing open with the seam allowance folded to the facing side. Add a stay stitch down the edge, stopping where the seam step starts.

Place both crotch seams together RIGHT SIDES TOGETHER. You’ll need to put one leg inside the other for this part. Match the front steps together and pin from the bottom of the fly all the way around to the back.

Stitch the crotch together from the back right up to the crotch step with a 1cm seam allowance and then immediately overlock this seam. Make sure to keep the zip guard out of the way when sewing this section - it can be a little fiddly but take it slow.

To secure the second half of the zip, lay the fly area flat with the left front overlapping the sewn zip side by at least 0.5cm. Reach inside and while holding the fly facing down, peel back the rest of the trouser and then pin the loose side of the zip to the fly facing without pinning the zip guard. Once you’ve pinned it lay everything flat again to check the position before sewing.

Once the position is confirmed you can then stitch the zip to the facing using the zipper foot to get close to the teeth. Make sure you only stitch through the zip and the fly facing and not the trouser leg too.

Start at the top of the zip and finish at the bottom of the stopper, doing a good backwards stitch to secure it.

At this stage you can also add an optional row of flat binding to conceal the zip tape for a high quality finish.

You’ll need to trim the width of your binding before starting. Cut down one of the folds you previously pressed to take a quarter of the width off your remaining binding.

Lay the binding down on top of the zip and line up the raw edge of the zip tape with the raw edge of the binding. Stitch a line on top of the previous stitching sewn to secure the zip.

Then fold the binding so that the free raw edge meets the line of stitching. And then fold it once more so that the whole binding is flat and covering the zip tape with all the raw edges concealed inside.

Edge stitch to secure and let the binding run all the way off the top and bottom.

Now lay the front trouser flat and pin the facing flat against the front leg. From the inside of the trouser stitch around the curve of the facing. It’s easiest to start at the top and carefully make your way down around the curve, going nice and slowly to get a smooth stitch line.

Sew right up to the crotch seam and do a good backwards stitch to finish. Make sure all other layers are pushed out of the way as you reach the front crotch seam.

You can choose either 1 or 2 lines of stitching around the fly (1 lines shown above). Your first line of stitching should be on the outer edge of the binding and the second line of stitching should be in the inner edge.

You can now finish your fly by stitching from the right side of the trousers.

First make sure your seams are pushed towards the fly curve.

Now from the top side of the trouser add a bar tack at the base of the fly opening. To do this, position your trouser sideways in your sewing machine and drop the needle right in the middle of the front crotch seam. Stitch forward and backwards by 3 stitches on the same spot to create your bar tack.

Then leave the needle down at the outer edge of the bar tack, swivel the trouser to the right and continue your 0.7cm topstitching around the crotch seam, making sure the overlocked seam is pushed to the right and sitting underneath the topstitching.

Lastly, from the top of the trouser add a discreet bar tack to secure the zip guard to the trouser. This will ensure this guard doesn’t flap open and stays in place. Trim all the loose threads and give the area a good press.