A new tutorial on calculating the Sum/Difference Frequency Generation (SFG/DFG) response is available on the Yambo Wiki website. This tutorial requires a deep knowledge of real-time simulations with Yambo and works with version >= 5.3 and the latest YamboPy.
A simple example on monolayer hBN is presented in the tutorial.

Here the wiki-page:

https://wiki.yambo-code.eu/wiki/index.php?title=Sum_frequency_generation