Don’t worry too much if your napkin is not a perfect square, it won’t matter in the end, I promise. You do need to ensure that your napkin is stiff however, so either use a napkin made of really heavy fabric, or use extra starch when you iron your napkin flat.
Start by folding your napkin in half, to make a rectangle.
Fold again to form a small square, and then turn it so that it looks like a diamond.
Fold the first layer up, but not all the way to the top. The do the same for each subsequent layer, just like in the photo.
Carefully turn the entire thing over.
Pick up the left side and fold it in a straight line across the top, as in the photo.
The fold the right side over so that it makes a point at the bottom.
Smooth the point out.
Flip the napkin over, so that the point is at the top.
See how the five squares sit on top of each other? Pick up the top one, and fold it up to meet the point.
Do the same for the next one.
Then you just tuck the point of the triangle underneath the top triangle.
And just keep doing that all the way down, including the bottom one.
And voila! You have a napkin Christmas tree.
You can of course get fancy, and add a chocolate or chalkboard star to the top so that it can double as a place setting; or use a patterned napkin instead of a plain one.