Thread a double strand of cotton onto a needle and knot and the end. You will need more thread than you think, so cut a generous length at this point.
Mark where you want the belt loops to begin and end, above and below the side seam, with two pins.
On the underside of the top marking, take two small backstitches, then pass the needle through to the front of the garment.
Bring the needle down to the bottom marking, allowing for some extra slack so that the belt loop isn't tight against the fabric. Sew a few of these loops (I did it four times) and each time allow the same amount of slack.
Blanket stitch over the loop you created, pulling the stitches tight against each other. This is why you needed to cut a long length of thread in step 1.
To finish, secure the thread with two backstitches on the wrong side.