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With a flexible measuring tape, like one you would find in sewing kit, take a measurement around your head. If you don’t have this kind of measuring tape, a piece of string will work. Write down the number of inches your head measured. If you used string, make a mark on it noting the measurement. You’ll use this measurement to create the band of the cap. If you want to make sure the cap isn’t too tight, you may want to add a little extra length. Cut the strip to your head measurement length and about 2 inches wide. After you cut it, circle it around your head to make sure it’s the right size. Trim it down or cut a longer strip as needed. Staple the strip into a circle. A cereal box is a good option for this step. With the cardboard circle sitting on the table, or counter, place a coffee filter inside it with open end up. Place 4 to 5 staples around the edge securing the filter to the cardboard circle. This wrap should extend about ½ inch past each side of the cardboard. Fold the extra down over the rim of the cardboard on the top and bottom. Then glue it in place to make sure it stays folded. You want the hat to appear all white, so try to make sure the cardboard is not visible. To do this, either use thick enough paper or fabric, or considering making it double thick with two layers. If you used a white shirt to cut out the collar, you could use more fabric from this same shirt. The portion of the coffee filter that you stapled to the cardboard will be the top of the hat so the staples don’t rest on your head. Allow all of the glue to dry before placing the hat on your head.

Summary:
Measure your head’s circumference. Cut a strip of thin, flexible cardboard. Attach a coffee filter to the cardboard circle. Wrap white fabric or paper around the cardboard. Push the coffee filter through so the bottom becomes the top.