Lay all components out on a flat surface or fully supported on trestles if available.
Mark out the crossmember positions and fit into position. Check the frame is square by measuring diagonally from one corner of the chassis to the opposite corner. Once both corner to corner measurements are identical, sandbag, clamp or secure the frame to prevent any movement.
Tack weld all the components together while constantly checking the chassis for squareness. The chassis is fabricated upside down, so all components (except the light channel) will be flush with the top of the chassis rails.
The light channels top face (when right way up) should be flush with the bottom face of the crossmembers as shown below.