With all the pieces cut, you can start assembling your frame. Bevel the joints on the side rail pieces to about 45 degrees, about half the wall thickness. Lay out one side on a flat surface, making sure the front side rail and the rear side rails are parallel. To check this, position the other front side rail above the rear side rail and measure the distance between the two.
Clamp down the side to be welded and recheck your measurements. When you are satisfied that everything is where it should be, tack the three pieces together. I prefer to tack on the top and bottom sides of the tubing so the tacks don’t get in the way when I tack the other side. Check to see that nothing moved during the tacking. If it looks good, lay the other side on top of the first side, check the dimensions and clamp the two sides together. Be sure the second side lines up perfectly with the first side and tack it. Now check to see that nothing moved and that the tacked sides are flat. Now weld the 1-1/2” sides of the tubing.
Set the two side rails on edge so they are level and parallel with each other. Now tack the rear crossmember to the side rails. Tack on the inside corner, it makes it easier to adjust the squareness later.
Now for the front crossmember. Slide the two spring perch gussets onto the tube and position it between the rails. If there is a seam on the tube, place it down and to the back. You can now tack the crossmember to the side rails, again on the inside corner.
Cross measure the frame to be sure it is square. I can’t stress how important it is that you keep the frame square at this point. I strongly suggest you tack tubes across the frame once everything is where it should b. Weld it up, skipping around so as to not pull it out of shape.