To make a code on a code stick, you need a long strip of paper, and a walking stick or a broomstick.
First you wind the strip of paper tightly around the broomstick.
Then you write your message all the way down the stick (you can write it on several lines).
When you unwind the message, no-one can read it unless they have the same stick as you! But if they have the stick, they can read the message just by winding the strip of paper around it again.