Design 3D Shapes: The octahedron!
Choose a size and make 1 Square Base, 2 Side Faces and 2 Closing Edges in the same size.
Check the diagrams, read all the steps before joining any pieces, and be sure to understand how it works. You can use pieces from other steps to help yourself check the geometry and join with the right angle.
Attach two pieces of Side Faces to the Square Base at A, B, 55º angle between faces, with a small bead; you can check the angle by joining them at the same time.
Turn the structure upside down and repeat step 3 on the other side of the Square Base.