You just need to sing different Mixtapes preferable as best as you can to fill up the value for the achievement based on your results. You need to do this on different songs and cannot replay songs for the achievement, but you have plenty of room for errors as you have to only achieve 50% of possible awards.
WARNING TO BUYERS OF USED RETAIL GAMES
Achievement may be locked and unobtainable for you!
There is a BIG WARNING to all those considering buying the retail version of the game probably used. At least for the German edition there is a download code included which is used to download in that case the German songs. BUT in addition you NEED to use the code to unlock the Mixtape game mode needed for this achievement!
In other words: there is no way to get this achievement, if you buy a used retail copy where the download code is also already used!