First I used a strip of fabric 11"x 60" it doesn't have to be perfect and you can change it to your taste. You can see there is a notch in the top of this piece. It didn't bother me since I cut up the end anyway.
I used a straight stitch with the longest length to run down each side, folding over the fabric by about 1/4" or so. I didn't worry about pinning it, I just folded it over and held it in place with my finger as I sewed along. It didn't need to be perfect.
You can see the seam here.
Make sure that you sew with the raw edges facing in on the same side. You don't want one side up and the other down. About 4" from the bottom I sewed a zig zag stitch to keep it from unraveling too far once I cut it. Completely optional but I thought it might help with the longevity of the piece if they wanted to keep it for dress up later.
Make sure that you do both sides. Then take a pair of scissors and cut the ends into strips. I found if you crunch up the fabric when you cut it makes a nice ragged edge.
After both edges are cut you are done!
Voila, happy pirates!