I made this bandana cap because I was asked to make something to wear under my farming hat.
All you need is a bandana, elastic, and a sewing kit.
I think a bandana of about 50 cm per side is good. (I used a 45cm and a 50cm bandana, both of which could be worn by adults.
You can easily do this by hand or with a sewing machine.

How to make it roughly
0:00 Materials, etc.
0:14 Sew through the elastic.
6:16 Sew the back
7:59 Sew the front and you're done!