This one's awesome because 1) it's so simple and 2) it can instantly transform a regular white or black top into a pirate costume. In fact, I may make one for myself, so I can dress up with the girls (with minimal effort! bonus!). In these pics, the belt is over the Puffy Pirate Shirt.
You can use any kind of fabric, but I used felt and my striped pirate knit fabric.
Measure the waist of the wearer. Subtract 2 inches from that and cut 2 rectangles of your main fabric that are that width by 5". For this belt, my wearer's waist is 20", so my rectangles are cut 18"x5".
Round out the corners on both rectangles.
Cut a strip of your coordinating fabric that is roughly 4 times your waist measurement by 3". This doesn't have to be exact, but you want it long enough to ruffle slightly. You can sew two or more pieces together if need be.
Fold your strip in half and press. Then run a gathering stitch down the raw edge. Ruffle till it will go around your entire main rectangle with a little overlap.
Sandwich ruffle in between your two main pieces, lining up all the raw edges. Pin. Overlap the ends of the ruffle piece. If you want to fold over the ends, so no raw edges are showing, do that.
Sew around the rectangle, leaving an opening to turn it through.
You'll have this. See my pins in the top left? Those are holding the opening closed.
Flatten that sucker out and topstitch around the edge.
Attach your grommets at the corners of both sides of the belt, following manufacturers instructions.
Lace ribbon or small rope through, and you're good! :)