I had a book years ago that showed how to fold 6 squares of paper and make a cube. With an extra couple of folds, the cube could stand on one point. I went a little farther--just for fun.

Step 1: Basic Cube

6 squares of paper--fold all the same way.

Fold paper in half so that you get 2 rectangles.
Open it up.
Fold in half the other way so you have 4 squares.
Open it up again.
Fold the 2 sides in towards the center crease.
Do NOT open it up.
Fold the two ends in towards the center crease.

Assemble by tucking the outer flaps inside the box. Use the pictures to help you with the assembly.

Step 2: Making the Cube Stand on a Corner

Make 6 pieces as for the regular cube.
Three of the pieces need the extra 2 folds.
Unfold one end.
Fold diagonally as shown.
Then fold the acute corner over to align with the other corner of the square (see picture).

Step 3: Building the Tree

Make 6 cubes--each with an inverted corner.
Each cube should be a different size so you need to cut your paper in the following sizes (6 sheets in each size):
8 inches
7 inches
6 inches
5 inches
4 inches
3 inches

You can decorate the paper before or after you build the cubes. If you are gluing decorations on the cubes, you should build it first--so that the decorations do not interfere with the folding or stacking process. You can cut your squares from decorative paper (ie. gift wrap). I sometimes cut the smallest squares from gold paper so that my tree has a star on top.