Okay, prep time. The first thing you'll need is two sheets of construction paper, one green and one white (or whatever other colors you want your tree and background to be).
Fold the green sheet in half lengthwise.
Cut out the shape of half of a Christmas tree on the fold. I freehanded mine, since it's a pretty simple shape. But if you'd rather, you can search for "Christmas tree pattern" on Google images and come up with lots of options. Just print one out in an 8x10 size and cut around it on top of the green paper.
Next, unfold the tree and use stick glue to adhere it to your white background. Be generous with the glue, and make sure to hit the edges and points well. I recommend using a scrap piece of paper underneath the tree.
Let this dry for a few minutes before you start the project with your child.
The other items you will need are tissue paper squares. I used red, yellow, and white tissue paper. Fold a sheet of tissue paper up and cut on the folds until you have approximately 2 inch squares (this is the part that is slightly time consuming!).
To make the ornaments for the tree, you'll take these squares and crumple them up into little balls. You can do this as part of the prep if you'd like, but I just made the balls as we worked.
Find a good work surface that your child can easily reach. I took the tray off of Sugarplum's highchair and let her work at the kitchen table. Place the tree in front of your child, hand them an ornament (yes, they may try to un-crumple it!), and let them place it wherever they want on the tree. Then use the glue stick to stick down the ornament in that spot. Another alternative, particularly for slightly older children, is to let them point to a spot on the tree where they'd like the ornament to go.
You could try using regular Elmer's glue as well, but I didn't want to deal with wet glue and a toddler at the same time. The stick glue worked perfectly well for us.
Be sure to write the child's name and the date on the back, then hang their masterpiece for everyone to admire!