Commonly called both the Preliminary and Square Base, the base is very common. Some of its uses include the formation of the Bird Base and the traditional iris.
The paper here is 15cm x 15cm, single-sided origami paper. Any paper cut into the shape of a square should be usable.
Turn the paper over.
Fold the top down to meet the bottom edge. Crease then unfold.
Rotate the paper 90°.
Fold the top down to meet the bottom edge and crease.
Unfold.
Turn the paper over.
Bring the top left corner to the bottom right corner. Crease then unfold.
Bring the top right corner to the bottom left corner and crease.
Unfold.
Turn the paper over.
Rotate 45° so that a corner of the paper points towards you.
Bring the right corner to the bottom corner. Use the previous creases as a guide.
Bring the left corner to the bottom corner.
Bring the top corner down to meet the others.
Flatten the model. It should be in the shape of a square with the open side towards you. You have completed the Preliminary/Square Base.