How can I make a custom seat pose? (Like in those vibe games)

I've heard you need to make an animation, but the scripting is where I get stuck.

Well, first of all, you should probably read up on Animation article. I'll link it below

I suppose, what you could do, is put a script inside the seat, and do a Humanoid.Seated function, and play the animation onto the character in that function.

Animation Article