I am planning on doing this activity with my boys tomorrow with our Christmas Tree Activities.
First you need a template or draw one up of a tree. Preschool Express has a tree pattern that I used.
(Note: I decided after I had printed this off that using green paper would have been smarter because if the paper shows through it wouldn't be as obvious with green background- or whatever color you make your tree)
Next glue the torn pieces of paper onto your tree.
Once you have done that you can be finished or you can help your child (if needed) cut out the tree like I did above. You could have your child glue it to a piece of cardstock, construction paper or make it into an ornament like I have pictured below.