Start with a piece of lined paper. Printer works too, but lined is easier.
Fold paper in half, hot dog style.
Fold in half again, hot dog style.
Fold in half, top to bottom.
Unfold last fold. A crease should appear in the middle.
Take the bottom and fold to the left until the line.
Fold bottom half upwards.
Fold left strip to the right
Turn it over again.
Tuck the little strip on the right into the triangle.
Fold top part without lines downwards.
Tuck the top into the triangle. Now it should look like a square.
Tuck the half into the triangle and you're done.