Just Dance 3

From the main menu, go to Dance, then Specials, and then go up to Simon Says. I don't know if you have to do anything special to unlock it, it was there when I looked. What I can tell you is that it will play a random song during the Simon Says mode, so start of by quitting and choosing it again until you find a song you think you can easily five star. Once you've done that, just play the song pretty much as normal, except for the extra 'Simon Says moves' they throw in every so often, appearing on the left side of the screen:

Jump - It'll show a little picture of a person with an arrow pointing up.
Spin - Picture with a person and a curved arrow, meaning you need no spin around.
Stop - Picture of a stop sign. Stop dancing to get awarded for this one.
Shake - Picture of a guy going wild. Move your arms and/or legs around a bit.

If you don't get the five stars, you can hit replay after the song ends and play the same song again. It seems as though you are awarded more points for hitting the Simon Says moves. I did a couple near perfect runs, only missing these, and barely got four stars. So try to hit them. But beware of the stop move... it seems nearly impossible to hit, so you may have to try and get lucky and get a run with less of these. They are random. The jump, spin and shake are simple enough if you're ready for them.

The achievement for this and getting three and four stars during Simon Says will pop as you're dancing, when the requirements have been met.