Ninety percent of all welding is really down to fitting. If pieces are fit and cut carefully, the welding process is a snap. If they're fit poorly, or there are large gaps, it can consume a lot of time, welding rods, and patience to get a good weld. I'm going to go through and deliberately make some mistakes to show you what I'm talking about. As you can see, at the very front edge of this weld, it's still possible to join the pieces. But as I continue welding, you can see that eventually the weld refuses to close. On the other hand, on the seam to the right of this last weld where the pieces are joined smoothly, it's possible to make a nice, smooth weld evenly shown.
You can see as I clean the weld, the difference in their appearance and also their strength. Where the material is fit closely, the weld is smooth and designated by these even ridges. As the weld gets wider, you can see that less weld material is deposited, and the material fails to join. After fitting, the most important thing to make sure that your welds are strong and that your forms turn out the way you planned, is to make sure that they're clamped securely to the table or to themselves, because heat will distort your welds. It will cause flat surfaces to buckle and long surfaces to twist.
Here you can see the difference between two welds, the top in which care was taken to support the work and clamp it to a flat surface, and the piece below in which that care wasn't taken, and the distortion is obvious. What you have to remember about metal is it heats locally as it expands. And as it contracts, it has nowhere to go, and therefore the metal around it takes up that stress, causing it to bend, warp, or otherwise distort.