Once they are nearly frozen, but not too solid, form into balls. To hang them either use a pre-made wreath feeder at Early's $16.99  (you can also make one using a slinky toy.) or attach a string. If you have brought them inside to make the balls or if they have thawed too much and become soft, let them freeze again before attaching the string.

To attach the string from which to hang them, cut about a 2 foot piece. You could use ribbon or twine or jute. Ones made with ribbon would add a festive touch. Wrap the string around the ball once and tie, leaving a little excess to tie each time.  Wrap again and tie as many times as you can, but making sure to leave enough string to tie to the tree.

Make sure to keep the balls frozen if not using right away. Makes a neat gift.

Info from Pick A Pepper.