GMing
GMing is an essential, fun part of the game. As a GM, you are in charge of keeping the game moving and of having fun yourself.
You are a player, too.
Never forget that.
RAW or ROC
I tried to outline as many rules as I could for COSMOS. If you are reading it right now on 27 Dec 2023, you know that it’s not finished. But that’s OK. Nothing is instantly finished anyway. What’s important is that the rules I’m providing - the Rules As Written, RAW - can provide you a basis for getting a game off the ground.
Now let’s say you’ve played the game through once and decided some of the rules are stupid after trying them out. By all means, come up with your own rules and ideas and implement them! The Rule of Cool - or ROC - can take precedence at your table if you decide it. But it’s up to you, the GM. Make sure you talk to your players before you start if they’d rather RAW or ROC to take the lead so you’re all on the same page.
Homebrewing
Do it.
How do I GM?
That’s a loaded question. The best way to learn to GM this system, or any system, is to play it first as a player. Barring that, you can attempt to play COSMOS single player - good luck! - and figure out what works or doesn’t or how certain rules work out.