How do I generate a ‘midi data file’ for the song I am playing?

Here is how to Record MIDI data in the SeeMusic app:

https://www.youtube.com/watch?v=QdZbZW-hO94

You can also record MIDIs in an outside recording app like Logic X, and then Import them into SeeMusic.

MIDI Files can also be downloaded for free from the internet – just use Google to search.