That is quite a broad question srrk!
We are designing the car on computer using CAD (or Computer Aided Design). We don’t use wind tunnels like you might think either, instead we using a very sophisticated technology called Computational Fluid Dynamic (or CFD for short!). This is where we get a supercomputer to to calculate all of the areas of high pressure and drag and lift on the car using several mathematical equations… think of it as doing 5 simultaneous equations at once for every single centimetre of the car!
If you want to know anything more in depth… just ask! 😀