Using a pj pant pattern I drafted a while ago and a 50c thrifted pillowcase, I shortened my pattern to make a cute pair of boxer shorts for Miss 9 adding a Faux Fly.
The pattern add on for the fly is quite simple to do yourself.
After washing pillowcase, open up at the seam to create a fabric piece.
When creating the fly add on pattern piece, ensure you allow for the seam allowance and waistband.
It should be a continuation of the waistband fold line.
The pattern piece you are using is for the front and it's not likely to be straight as it's the inner crotch side.
Allow the pattern piece to follow this slight curve as best as you can, keeping the proportions the same.
The size of the faux fly can be any size you want. Mine is 11cm x 3cm when sewn - the pattern has a 1cm seam allowance which has been added.
Once you cut the pattern you will have two odd looking fronts like this.
Pin your right sides together. I have quickly overlocked my edges. Sew along your seam allowance to the crotch point where my last pin is.
Once sewn, turn your piece over and the fly should naturally flop to one side. I know there are certain rules for which side the fly is sewn to but I don't think it matters.
Once you have folded the fly piece over into it's correct position and it is even from top to bottom, give it a good press. When you turn it over to the right side it should look like this.
Pin in place and sew either one or two rows of top stitching from the right side of your fabric.
Keep your pins in place and finish off your garment as per your pattern instructions. When finished, the waistband will hold the fly closed or you can add some buttons as I did.
Another option if you don't want the fly open, is to run an edgestitch from the top waistband along the centre seam all the way down before sewing your pieces together.