Metal shaping is an invaluable skill to have in hot rod fabrication. There are many specialized tools used in the forming process.
Shaping metal is basically as simple as shrinking or stretching it to achieve the required shape.
 Bucks and patterns
Before any metal can start forming, its good to know the exact shape of the piece one wishes to create. Bucks can be made using anything from styrofoam to plaster and anything between that a pattern can be pulled from. Below is a picture using "Burt" buck strips of metal and foil tape to create the shape required. Chicken wire was used to hold the metal strips in place in order to tack and tape them together. The buck was then slathered with bondo to check the actual shape and to pull the best possible pattern from.
Once the buck is ready, a flexible shape pattern can then be made. The pattern starts with low stick masking tape. Plastic stretch wrap and transfer tape can also be used.
The low stick tape is then covered with fiberglass reinforced packing tape applied in two directions. This helps it maintain its shape when removed. Once the pattern is ready it is pulled off carefully and then the back is dusted with baby powder or similar item to stop it from sticking to itself. Reference marks can be added to the pattern before adding the packing tape. The pattern can also be cut into manageable sized pieces before removal from buck.
 Using the pattern
The pattern can be placed on the sheet stock and traced around for the rough cut. Adding an inch here is a wise decision as the piece will be shrunk and stretched so having a large enough piece is crucial. However, hand shaping metal is quite a workout on the arms so having too much extra material can be a waste of time as you will be shaping metal that will be cut off later.
With a bowl shaped piece the edges can be shrunk on the stump first. The pattern is then placed on the part. One then pushes down on the flexible pattern to find hollow spots underneath the pattern. This is the key to the pattern. Finding the hollow spots below tells you that either those areas need to come up more or that the edges need more shrinking.
Shrinking metal can be done using a stump and wooden or plastic mallet. A set of tucking forks can also be used but this process can be slow and tedious. A reciprocating machine such as a Pullmax with thumbnail dies can also be used to shrink metal rapidly. This is not a machine for a novice though. Learning to shrink metal by hand is said to be the best way to see how it reacts throughout the process. In the photo below is a "stump" made from pieces of maple hardwood glued together and then different sized cone shaped cutouts made.
Stretching metal can be done on a shot bag with a wooden, plastic or steel mallet. The bag is made of leather and filled with a material such as lead shot or sand. Plastic injection molding pellets have been used for this as well. Metal may also be stretched over a hollow such as the cut out of a stump or even a piece of pipe. In the photo above, one corner of the stump has a 3" dia hole for stretching metal. Shrinking a piece of metal around the edges will result in a bowl like shape. After shrinking the edges to the maximum amount allowed by this process, it then becomes necessary to stretch the center area of the piece to achieve the radius required.
 Reverse curve
Stretching the edges of a panel only and not working the center at all will achieve the classic potato chip shape. This is called the reverse curve. The stretching can be done by holding the edge of the sheet over a cutout in a stump and applying some severe hammer blows. The photo below shows a wooden mallet with a small radius and a panel that has been stretched over the 3" dia cut out.
The photo below shows both edges of the panel after a lot of stretching over the hole on the stump. The edges of the panel were held half over the hole and the edge of the sheet driven into the hole for stretch also.
 English wheel
An English wheel (ewheel) can then be used to smooth out the lumps created from the stretching or shrinking processes. If too much pressure is exerted by the ewheel on areas that were shrunk, the material will start to stretch changing the shape. If too much pressure is exerted at the stretched areas, the metal will be stretched even more and change the shape. Picture of the ewheel below. This is a clone based on the Imperial Wheeling Machines brand.
A good quality wheel and anvils will greatly help in the wheeling process as the contact between the rollers and sheet will remain constant and give nice results.
The photo below shows the same panel after more stretching followed by rolling it thru the ewheel to smooth out the lumps.
 Hammer forms
A hammer form is usually two pieces of wood or other materials used to sandwich a piece of metal in order to bend it to shape. Body filler and cement can also be used. This technique can be used to form complex shapes.
In the photos below, high density polyethylene sheet was used along with a 1/8" thick steel plate. This panel could have been done on the bead roller but was done with a hammer form instead for a more accurate shape. The plate was used to achieve a nice defined edge.
The top piece of plastic is cut on an angle to allow for hammer clearance. The pieces in the photos below are arranged in the order they will be bolted together.
The hammer blows in the photo below are directed almost straight into the material with a slight downward trajectory. Tapping the edge into itself allows the material to shrink. This was done all the way around the sheet.