These instructions are a work in progress, and we haven't created the illustrations yet. They will be added soon.
Put your two backs on top of each other with the good sides together, and sew them together allong the center back seam.
Put two of your fronts on top of each other with the good sides together, and sew them together allong the center front seam.
Repeat with the two other fronts so you have two identical sets.
We have 4 fronts because we'll have a double layer at the front of our swim trunks.
Since swimwear fabric tends to be slippery, it's a good idea to baste your fronts together to make it easier to work with them in the next steps.
So place your fronts on top of each other, with good sides out, and sew them together with some large basting stitches in the seam allowance.
Place your back down with the good side up, and place your double front layer on top. Sew fronts and back together at the side seams.
Keep your swim trunks with the good side inwards (the back at least, the front has good side on both sides) and sew the crossseam to create the leg openings.
Mark the middle of your waistband length. Fold the waistband double, and mark the middle of the width (do not take the seam allowance into account).
A bit to the left and right of this, you can add two eyelets to pass a drawstring through. This will avoid an embarassing situation when you go for that big dive and now suddenly find your swim trunks on your ankles.
As swimwear fabric is slippery and thin, you might want to add some reinforcement behind these eyelets. A leftover piece of denim will do just fine.
There's no magic formula for the length of your elastic. So you wrap it around your waist and pull it tight until you get a good fit.
Mark this length, cut the elastic, and join the two ends together.
Fold the waistband in half with good sides together and line of the (short) edges. Sew them together.
Fold your waistband double along the length with good sides out, and place the elastic inside. Make sure to align the place where the elastic is joined with the place the waistband is joined.
Align the back of your swim trunks with the place where you joined the waistband, align the edge of the waistband with the edge of the back, and pin them together. Make certain that you pin it to the bad side of the back.
If you've made eyelets in your waistband, double check that they are placed towards the outside, not the inside of your waistband.
Find the front of the waistband (easy if there's eyelets, if not just fold it double) and align that with the front seam. Pin this in place too.
Now make your way around the swim trunks waist, pinning the waistband in place.
Then, sew the waistband to the swim trunks, as close to the the elastic as you can, but don't sew into the elastic.
It's fine to not sew too close the first time around, and once your elastic is attached and encased, make a second round to sew it a bit more snugly.
Fold the hem upwards, and sew it down. If you have a coverlock, use it. If not, use a twin needle or zig-zag stitch to keep the seam stretchable.
Swimwear fabric doesn't ravel, so you can simply fold this over once and sew it down, then neatly trim back the fabric.
Thread a drawstring through one eyelet, around the waist, and out of the other eyelet.
That's it, you're done 👍