Once you've cut everything out as accurately as possible (see Part 1), you need to lay out your work on a flat surface. Here Travis is using a 10mm steel plate, but anything flat (even a concrete floor) is preferable to trying to do it "by eye."
Professional fabrication shops will often have a ground flat cast iron table. It's worth looking our for old machinery being scrapped, as an old planer or milling machine bed can make a fantastic layout table.
After the parts are laid out, we tack weld every other joint.
A good process for welding a square, hexagon, or other geometric shape is to alternate tacking the outside and inside corners, and check the work frequently.
As we get farther along, it's important to keep the whole assembly from warping or getting out of flat. Restraining the parts in at least two places prevents this. Here we are using a standard 'C' clamp on the right and a fancy welding clamp on the left.
This all seems like a real pain in the rear, but you will thank yourself later, when everything has been covered with hundreds of inches of weld beads and you realize that all of those little errors add up.
Coming soon: part 3.