Bevel the edges of all your joints to get maximum penetration, and if you are not blessed with a frame table - and I am not - measure, measure, measure. Get everything level and clamped together, tack things, then measure again. For the rear section, I tacked the pieces of one side together, then clamped the pieces of the other side to it and welded them up on one side, dressed the welds and swapped sides to finish welding them. That made it easier to attach it to the main frame and get it square & level. I used 2x4's and bar clamps to help keep the rails spaced right and just about every other clamp I have to get it all square and positioned right. It's like most things, the quality of the results are dependent on the quality of the prep.