Shouts out to the players who actually give the GM feedback. I got 1 in the group and it is a breath of air. They tinkered with the new Licenses I made and so far they like the concepts of both, so we have a solid direction.
I'm a player who gives feedback to my GMs. I've been told, by my friends, that my feedback made them a better GM.
So, how to give feedback.
It's basically answering these questions to the GM.
What did you enjoy about the session, specifically? Character voices? The battle map? Doing something different? (Maybe you were on mapping duty and really liked that.) The NPC interactions? The story, the freedom, the freedom to choose the direction of the story?
More importantly, imo, what you didn't enjoy.
What grinded your gears? Did you feel railroaded, or directionless? Was the boss too damage-spongy? Were the enemies not interesting to fight? Did you feel your character choices only made the session untenable to express yourself?
What are you excited for in the coming sessions?
Are you enjoying the system, if it's a new one?
Of course there's more, but that's a solid start.
What you liked
What you didn't like
What you're excited for.
Even a simple, "I liked the robotman with the funny cube" and that's it, is gold to a GM.