How do I make it so players can see less?

Like putting a frame so they only have a small circle to work with, and making the game really dark, I wanna make a cave setting for a game I'm working on, but I have no clue how to do it. I've already looked at this: https://scriptinghelpers.org/questions/27555/how-do-you-darken-a-players-screen#google_vignette but due to my lack of scripting experience for the frame part, I couldn't get it to work, could anyone give me a way with scripts or plugins?