Scratch-building
Scratch building is easiest if original plans of the subject exist; however, many models have been built from photographs by measuring a known object in the photograph and extrapolating the rest of the dimensions.
The necessary parts are then fashioned out of a suitable material, such as wood, plastic, plaster, clay, metal, polymer clay, or even paper, and then assembled.
Some purists consider a model not to be truly scratch-built unless all of the parts were made from raw materials. However most modelers would consider a model including commercial detail parts as scratch-built.
Scratch-building a new body onto an altered ready-to-run chassis is also acceptable!