The preliminary base, or square base, is one of the two most commonly used origami bases, along with the Water Bomb base, it’s inverse. It gets it’s name from the fact that it is used as the base for so many models, and even other bases.
Once folded, the preliminary base is shaped like a square with a central crease dividing it into to triangular sections, each composed of two triangular flaps.
create the crease pattern
Fold the northwest corner so it meets the southeastern corner, crease and unfold. (there should now be a diagonal fold running northeast/southwest through the center of the paper)
Repeat the last step in the other direction, folding the northeast corner to the southwestern corner, crease and unfold. (Now there should be two diagonal valley folds running northwest/southeast and northeast/southwest that intersect in the center of the paper)
Flip the paper over. (There should now be two diagonal mountain folds, running east/west and north/south, that intersect in the center of the paper)
Fold the northern edge down to meet the southern edge of the paper, crease and unfold. (Along with the intersecting diagonal mountain folds, there should now be a east/west valley fold running through the center of your paper)
Fold the eastern edge to meet the western edge, crease and unfold. (there should now be four creases on the paper, two mountain running east/west and north/south, and two valley diagonal folds that all intersect in the center of the paper.)
Collapse the pattern
Lift the portion of the paper south of the east/west crease so that it is perpendicular to the rest of the paper. (The paper should now be divided into two, main, north and south, rectangular sections at a 90 degree angle to each other, each section should be divided into two square quadrants which are farther divided into smaller triangles by diagonal mountain folds.
Fold only the southeastern quadrant along it’s dividing diagonal mountain fold into a triangular flap, that points inwards to the rest of the model. (If this was done correctly, the South southeast and southeast east sub sections should now be folded into a triangular flap, and the overall model may resemble half of a box, with a bottom and two walls, at the south and east, with the flap, created in this step, pointing northwest, at the intersection of the southern and eastern walls.)
Fold the two subsections bordering the triangular flap, the upper triangles of the southern and eastern walls, on to the flap. (The eastern part of the model should now consist of the four layered triangular flap, while the western portion should slightly resemble a upside-down pyramidal pocket, whose eastern and western corners are razed.
Find the western most crease of the pocket, push it in towards the center of the model; if you did everything correctly, a second four layered triangle should be formed. (The preliminary base should now be complete! It should consist of a square with a central crease dividing it into two triangular sections, each composed of two flaps.)