Step 3: Create a welding jig
Why is this necessary? When you weld pieces of metal together, they heat up and cool, shrinking in the process. This leads to some deformation in pieces with either many pieces or many welds. A jig seeks to prevent this by holding the pieces in place while welding, thus reducing deformation. Since this whole thing is supposed to be symmetrical and rest flat on the floor, a wobbly, deformed octagon would just be sad. Thus, use a jig.
Thankfully the base is a regular shape, so you can just make a jig for two joints and rotate the base around between welds. So to make this, draw out how three pieces of the octagon should theoretically join up onto your worktable (or, ideally, on a scrap piece of plywood so you don't later char your worktable while welding) (also, use a ruler/protractor rather than freehand), then lay three pieces on your drawing, fitting them together tightly to test your theoretical drawing. Then screw small wooden blocks around your pieces so that the blocks tightly hold your pieces all together. Make sure to leave room around the joints between pieces so that you can have access to weld. With the blocks securely in place, add two of the struts that fit inside the hexagon and add securing blocks for them. You're done!