Hi John,
If your form is to be symmetrical, you could hammer each half through a "plywood" cut out. (Substitute material of your choice for plywood). You would get a two flanged halves that you could trim and weld.
If your form were to be asymmetrical, your "plywood" would also have to be curved as you wish your form to curve. Weathervanes can be made this way.
When I work free hand without aids like the plywood, I prioritize work this way: Identify the shapes within the form first, remembering that a simple bend should not be mistaken as shape. A straight edge won't lay flat on a crown, so that's useful to know, but it will lay happily on a bend, and could lay transversely on a reverse. I like to put reverses in next, since I find an advantage to starting them in a flat sheet, and they contribute to the work needed to make the crowns. Fill in around the reverses with crowns lastly.
https://allmetalshaping.com/showpost...2&postcount=20 There will be some back and forth to adjust and fill out the form, but by working "hammer and dolly" with an "off dolly" approach you can adjust areas that need it readily with a little practice. When a gap appears at the weld-line in a reversed area, try reversing it more in that area; or in a crowned area, by crowning it more, causing the edge to drop closer to the weld line in each case. A lengthier description may be needed here, since various methods will produce similar results in different hands.
I made a bird body once by shaping both halves at once, by pop riveting two metal pieces together and shaping until they would lay flat on the table, then popping one of the two pieces inside out after the two were separated then welding them at center. It worked ok, but I never did it that way again
