Here is basic description of how to make castings using RTV rubber and Urethane resin.
1 I use photos, ORER, data sheets and actual measurements to produce CAD drawings of the cars. This helps to understand how the cars are built and also is essential when building the masters.
2 The masters are built from styrene sheet and strips. It's easy to work with, it comes in a variety of thickness and shapes and a minimal amount of tools are needed to work with it. I mainly use a scale ruler, a small square and a sharp hobby knife.
3 RTV rubber is poured around the master to get the negative impression of the part. After the rubber has cured and the master has been removed, resin is poured in to the mould, which produces the castings that are used for the models.
4
The castings have to be cleaned of any flash and  then the parts are assembled using ACC type glue.