For this tutorial, I have chosen fabric that looks different on the right and wrong sides. The right side is a blue and grey stripe, the wrong side is a tiny grey dot on a blue background.
Step 1: Cut out the pattern.
The main piece needs to be cut once, on the fold. The crotch piece needs to be cut 4 times, so cut it out twice through a double thickness of fabric.
Step 2: Sew the crotch piece.
Arrange the crotch pieces so that you have 2 sets, right sides facing in.
Place one set on top of the other.
Sew through all 4 layers at once, down the CF seam (on the pattern, this will be the un-notched seam). I have used an overlocker to sew this seam. If you do not have an overlocker, try a narrow zig-zag stitch.
Open out the top layer of fabric and the very bottom layer of fabric.
You should now have a double layer crotch piece with no exposed seams.
Step 3: Attach the crotch piece to the main panel.
Sew one side of the crotch panel to one of the curved (and notched, on the downloadable pattern) seams on the main panel, right sides together. You will be sewing through 3 thicknesses of fabric altogether.
Fold over the main panel and attach the other side of the crotch piece to the other curved seam of the main panel.
The trunks should now look like this, from the inside.
Step 4: Sew the inner leg seam.
Flip the trunks over and match up the inner leg seams at the bottom of the crotch seams to the inner leg "slit" on the main panel. Stitch, being careful to catch the sharp corner at the top of the slit on the main panel.
Step 5: Hem the trunks.
This can be done with either a cover stitch machine, or a twin needle on a regular machine, or a zig-zag stitch on a regular machine. I have just used a zig-zag on a regular machine. The zig-zag stitch allows the leg opening to stretch.
Step 6: Attach the waistband elastic.
The elastic is sewn with a flat joining seam to minimise bulk. Commercially, this would be done with a cover stitch. There are a couple of different stitches you could use on a regular machine. The photo below shows where I have tested out a few different stitches using a scrap of elastic on some scrap fabric. From left to right, I have tried on the the stretch stitches that comes on my machine, a 3-step zig-zag with short stitch length, a 3-step zig-zag with a longer stitch length and a regular zig-zag. Have a play around and decide what works best for you.
Stretch stitch on my machine
3-step zig-zag stitch selector.
You can use the recommended elastic lengths, given with the pattern. You could also custom measure, making sure that the final measurement is at least a smidge smaller than the fabric to which it is being attached.
Overlap the ends of the elastic by 2 cm.
Stitch to secure.
Divide the waist of the trunks into quarters. The easiest way to do this is to mark the CF (centre front) with a pin.
Flatten the trunks so that the CF pin is at one side. Place another pin at the other side.
Flatten the trunks so that these 2 pins are directly on top of each other. Put pins at the new sides.
Similarly, mark the elastic in quarters. I mark the elastic with a marker, rather than pins, as it is less fiddly.
Sit the elastic on the outside of the trunks, matching up marks on the elastic with pins in the trunks and aligning the top edges. You can either put the elastic seam at the CF or the CB. You could always sew a tag over the elastic seam if you did not want it to show.
Stitch the bottom edge of the elastic to the trunks, slightly stretching the elastic to fit the fabric, using the stitch that you decided above.
Turn the trunks inside out and cut away any excess fabric above your line of stitching.
Ta-da! Completed trunks.
Right side out.