Setting Marks for Actors

What are the rules for setting marks that this video mentions?  How can you begin using these suggestions immediately in your productions?

    Marks are a useful tool that can help improve how smooth a shoot can run regarding talent and actors. You are able to use them to show where an actor should be in a shot so that the framing and focus is maintained. The rules for setting marks are to use different colors for each actor in the scene, use a separate and dedicated color for focus or camera department marks, and ensure that your marks aren't visible in a shot. The options for what can be used as marks are tape, sandbag marks, and t-brackets. I can begin using these suggestions in my productions by preparing tape or designated marks for my next project and utilizing them for the actors.