At first, I advise choosing the size of the tube, which will continue to draw the size of the future of the cap during robots. large plugs give a result larger. small, such as lemonade corks, are more tiny hats.
So, the choice is made on the tube from the water and continue more …

By the size of the selected plug customize the size of the circle cut from. manufacturing large hats can use old CD.

According to cut a cardboard circle Measure out and cut a circle of fabric, large size hats bases:

to do decoration of the cap – according to your taste, desires, and capabilities:
Materials for decoration can be “planted” on the glue, sew or pin.