I think a prerequisite is to have some experience with functional programming (F#, OCaml, Scheme...) and then realise a simple option type implement a monadic i

I think a prerequisite is to have some experience with functional programming (F#, OCaml, Scheme...) and then realise a simple option type implement a monadic interface (bind + return). From there, you can basically try to write functions which return an optional value using bind/return rather than pattern matching on Some/None. From there, it comes with practice and you can see how bind/return work for other things (notably concurrency). There's no need for any theory to use monads effectively. To me, the problem with Haskell is that you need monad from the beginning, whereas in OCaml, you can start using them when you're familiar with basic ML.