Here are the different illustrated steps to create a pompom.

Step 1 : 

Wrap the wool around the two nested parts.

The more turns you make of the wool, the fuller the pompom will be.

Step 2 : 

Cut all the threads by inserting your scissors into the groove between the two pieces.

Step 3 :

Insert a piece of wool into the groove to tighten the different pieces of wool and also to create a loop to hang the pompom.

Step 4 :

Pull the pom pom out of the 3d piece set.

It's already done.