This is how we make parts when we have an original to use for a pattern. This is part of a turbocharger inlet scoop and diverter (not sure what its actually called). This is stainless steel and incredibly difficult to work with.
We fill in holes with Bondo to get it as smooth as possible. Then we put plastic food wrap on top of it and then cover that with masking tape. The tape is laid across the plastic in at least two directions to make it stiff. Mark the edges and cut it to fit and you now have a 2-D pattern of a 3-D part. Put that on a flat piece of aluminum or steel and trace it.
We start cutting it out by using a step drill for any sharp corners that would be impossible to cut with shears. Then we use hand shears to cut out the rest. Then the real fun starts. Basically, we make a resin female mold and pound the steel into it with nylon hammers. More on that later if people want to see it.