Princples of process (systems) based modeling
I guess the only thing the modeler should know is either the mathematical equation describing the system or the statistical relationship between a predictor and a predictand. You might have "access" to the exact equations or you might need to fit a mathematical model onto empirical observations. Then you need to code these mathematical or statistical relationship into a software (i.e. numerical model).

Fairly general. The devil will be in the details.