The key to this is to keep time with your trainer for one drill. This can be done in "quick start", "continue program" or "5 minute session". Some exercises are easier to do this on than others. As you get the feel for the game you will find the exercises that you have an easier time with and you will get more of a sense of the timing the game looks for on certain exercises. You WILL unlock this eventually if you workout regularly and continually focus on staying with the trainer's pace.
Be patient and keep trying. Sometimes the Kinect will tell you that you are going too fast or too slow when you know you are on pace. Eventually, if you keep trying, you will get it! The achievement will unlock at the end of the specific drill if you do it correctly.