I can’t speak from real life experience, but one movie that actually handles this really well (as far as I can tell) is The Quiet Man, during a fight.

There’s an example of an impromptu, casual bet between two individuals who are understood to trust one another, where they actually set the odds and agree formally, and it all happens very smoothly and naturally so as not to be boring: “Five to one on the big chap” “Given or taken?” “Given” “Taken” Handshake

There is also an example of the more chaotic, mass, unplanned betting, where a character who is already established to be a jack of all trades known to the community pulls out a notebook and takes on the role of bookie. I think they even show the odds being adjusted in real time as the fight progresses, but I don’t recall for sure.