I learned how to make these folded paper trees from a TV craft show several years ago, and I've made tons of them since. They're fast, impressive, and make great ornaments. Teensy mini trees are cute, too - all you need is a smaller square of paper. Double-sided origami paper makes fantastic trees, but any paper that will hold a crease will work well.
Start with your sheet of letter-sized paper and fold one corner to the edge to make a triangle. Make sure you crease the fold well.
While paper is still folded, cut the excess paper away from the triangle to make a perfect square. Unfold.
Now fold your square in half the opposite way and crease.
Your folds now ake an X. Unfold, then fold in half lengthwise and crease as shown.
Unfold again and fold in half one more time. Your square should look like this.
Unfold one last time and re-fold square in half from corner to corner.
Press the upper left corner of your triangle in along the folds.
Repeat with the other side.
This is what your square should look like.
Using a pen or pencil, to get a sharp point, flatten out each "leaf" of your square.
Your tree should now look like this, once you've flattened each side of the square all the way around. Now, mark your tree at the trunk like I have, and cut along the dotted lines.
Cut straight across the bottom.
Now, cut about 1/4" through all layers straight towards the middle of the tree as shown. Then separate the pages of the tree and fold each notch upward, one at a time. Repeat all the way around.
Spread the pages out evenly and, Voila! You've got a paper Christmas tree.