A friend and I have been contacted by someone to see if he could commission us to make a TTS mod for the prototype of his board game. We don't have any experience with the "business and legal" side of things involved in that so I figured I would ask here if other people already have done TTS mods as contract works, and if they have tips or experience to share on that topic.

Some context:
A friend and me have made a high-quality TTS mod for a space board game ([URL="https://steamcommunity.com/sharedfiles/filedetails/?id=478727953"]here[/URL]), with extensive scripts.
Now a few days ago, someone popped in the Discord server associated to this mod and its community, and said he would like to contact the makers to commission them to make a pre prototype of his own space board game in a TTS mod. His game is in development, and he said he got a loan for it and commissioned some artwork already (which he also posted to show he was not joking). He also posted a link to a concept document for his board game that describes some of the key concepts (like a 3D board made of multiple levels) but that clearly lacks polish (it doesn't say how you win ).
All in all it looks like he's very much unexperimented, wants to fulfil his dream of publishing a board game and already got started (getting the loan and commissioning art).

So if anyone already made some TTS mods as contract works, I would like to ask you how your own contract works unfolded and if you have tips or experience to share about them. For example, did you have a contract? If yes what did it look like? Did you have to do legal things (registering as a freelancer or something like that?)? What about the copyright that you automatically get on the things you make, was that a problem?

As you can probably guess, I don't know much about business and legal matters, so any help is welcome.
-- Cyrusa