An English Wheel or Planishing Hammer will form these fenders, but if you don't do a lot of sheet metal forming, and would rather avoid the learning curve, and expense of one of these machines, cold hammer forming is a good option for the occasional project. I looked around the web and found a nice article on cold forming of a classic flared fender.
To download this step-by-step article click Forming Sheet Metal # 1, 256 KB - pdf.
Above is a picture of the first page of the 12 page article. The article takes you step-by-step from a flat sheet below.
To this fully formed section of the fender.
This was all done cold working with hammers, sheet metal working chisels, and a wire form to guide the shape forming. My new 4' cold work RR anvil is perfect for this type of work. You may recall how I expanded the bottom of one of the doors on my band sawmill by cold hammering. This gives me the idea of another improvement, round off one end for doing circular work.