METSIM can be used at increasing levels of power and flexibility, with each level requiring more understanding of simulation methods.
From the simplest use of the standard units ops, to novel uses of general unit ops (such as distributors and splitters), and then the creation of customised unit operation models, METSIM with APL provides the complete range of required tools.
The mathematical "F-Functions" provide the most commonly used data manipulations that you should use and understand before you start using APL to try and do the same or similar data manipulations.
Studying the F-Functions will provide insight to what is required for simulation and how METSIM works. Then you are better prepared to customise (and hopefully enhance) your modelling skills with User Defined Functions using APL.