Step 1
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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.
Step 2
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Mark where you want the belt loops to begin and end, above and below the side seam, with two pins.
Step 3
On the underside of the top marking, take two small backstitches, then pass the needle through to the front of the garment.
Step 4
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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.
Step 5
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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.