Pocket square are to decorate the jackets (coats) breast pocket. Pocket squares can be made from silk, linen, cotton, wool, or other exotic fabrics.

Pocket squares can be folded in various ways. Go to this site for  detailed instructions on how to fold them.

http://www.samhober.com/howtofoldpocketsquares/pocketsquarefolds.htm

The golden rule in using a pocket square is “Coordinating,” and not “Matching.” Never use a pocket square that is identical to the tie you are wearing. Sometimes when you a buy a tie or maybe you receive a tie as a gift and it comes as a set with a matching pocket square. NEVER USE THAT POCKET SQUARE WITH THE MATCHING TIE. This is a serious style blunder. YOUR POCKET SQUARE MUST NOT BE IDENTICAL TO YOUR TIE.