Hi shamus, probably the easiest way is to start with using Draw curve > Rect to draw a square. Then use Construct > Extrude with the "To point" option to make a Pyramid solid from the square. Then use Transform > Mirror to make a mirrored copy of the pyramid along its base line. Next select both pyramids and use Construct > Boolean > Union to combine them together into an octahedron. To hollow out the interior of the octahedron select it and use Construct > Offset > Shell and enter in your desired wall thickness.

Let me know if you get stuck on any of these steps.

- Michael