When you are using a special note, they will usually come directly after the event you want to comment on. This can happen after a sequence or scene, but usually after a significant chunk has gone. You then indicate that it is a note and keep it in parenthesis. Example:
(NOTE: Though the character is explaining the rules of blackjack, and you will be able to pick out the details at times, he should be talking too fast for most people to understand the entire thing. You then use a CUT AWAY to show that the other characters cannot understand him either. This way the audience knows that they are not intended to understand everything.)
This special note would then go at the end of the monologue of the other character, yet indicate something about the speech that would not be clearly enough indicated by the actions of everyone in the scene.