Please refer to the reference book given in the link below.

https://onlinelibrary.wiley.com/doi/book/10.1002/9780470723876

Also start with the code available on the link you mentioned in your question by going through line by line.

Hope this will help.

A couple of good, free references are Inside PROLITH and Optical Lithography Modeling with MATLAB

http://lithoguru.com/scientist/litho_papers/Inside_PROLITH.pdf

http://lithoguru.com/textbook/labs.html

The first is by Chris Mack, the guru of litho simulation. The second is by Kevin Berwick

The code you referenced probably doesn't do anymore than a simple Fourier propagation. Any reference on Fourier Optics such as Computational Fourier Optics by David George Voelz will have examples of how to code this.

Have Fun.