Building a nonlinear model is expert work, nothing you can easily do as a beginner in a few weeks.
You can buy a library with nonlinear diode models, including some PIN diodes, from Modelithics.
https://www.modelithics.com/products.asp?a=view&id=2
This library can be used with Microwave Office or ADS.
Personally, I support the idea of using S-parameters. This is what most designers would do.
Your professor might be a bit optimistic regarding the nonlinear models.