Welding
You need to use the stitch method, because burn through can easily happen. Weld a tack about every .5" or so.
The outside, (pictured) is easier to tack than the backside, simply start your weld just below the seam and count to four. Spend a two count, (about 1 second) under the seam and run it up. At the 3 count you will be mid seam, at the 4 count you will be slightly above it. This takes about 4 seconds a tack. If you have a completely different welder, try and adjust till this goal is accomplished.
These little weldboxes aren`t known for their strength so if you have one move the ground closer to your work as you go.
After the outside is stitched get under and do the backside. You will notice you can somewhat see where the outer welds went. I like to aim to put the inner welds between the outers.
The back side is a little different method, since the seam now is reversed, and it`s tight for room. I ran each tack horizontaly, to try and fuse along the seam as opposed to across.
Up to you really, just remember to pace yourself and don`t burn through or it`ll upset you deeply and you`ll have to pour more snot in the hole to try and fix it. Once it blows through, it has a habit of wanting to keep burning away, so take it easy.