A band saw is by far the best way to cut sheetmetal; unfortunately most run too fast to cut steel, this one runs about 180 surface feet per minute, which cuts through 18 gauge like butter. I started this project with a Bosch jigsaw, which worked ok, but the band saw was a massive improvement in speed, accuracy, and convenience. | 
With the bend locations and direction marked on the part, I created smooth radius bends with a hammer and T-dolly clamped in a bench vise. A good bench vise and solid work surface is essential. | 
With the basic geometry taking shape a few passes through the shrinker added the just the right curve. This took about two minutes on the shrinker--this tool rocks. |
Once you get a belt sander you may never power up your bench grinder again, they work wonders for quick adjustments to sheetmetal parts. | 
To flatten out the end of the shape and maintain curve geometry, a very crude hammer form is made with a chunk of wood on the belt sander. Here you can see the part sandwiched between two pieces of metal, with the block of wood under the sheet metal part. | 
The block of wood acted as a form to work the sheet metal over, and with a little more work, will match the needed shape. |
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Another piece of the puzzle tacked in to the left of the piece we just looked at. Notice how the preceding part has almost disappeared. | 
While this part fits pretty close, tacking it in a couple of spots lets me further refine the fit with a screwdriver or hammer until it matches perfectly. It's ok if the part doesn't fit with surgical precision as long as you can bend it to fit once tacked in; a common trick is to work it with a screwdriver as you weld so the pieces remain lined up. |