When your little one asks you to watch tv and you say something like, "Ok, but only one show. And then you have to turn it off." And they respond with an ok. But will they really turn it off? Probably not.
The best way I've found to make sure they heard me and will do it is a little quiz. "Are you going to watch tv?" Wait for the Yes.
Then, "What are you going to do at the end of the show?" Usually, they can't answer. All they intially heard was, "Ok!" So, repeat the instructions. And start the quiz over.
When I can get my little one to tell me that he will stand up and turn it off, it usually happens! And of course it works for any kind of direction giving.
Painting: The little knitters by Albert Anker