The hardest part will be the transition when you are at the top of your pull up. To start, do this motion in reverse. Start by jumping up and getting yourself in the position that you would be in at the top of the muscle up, with your arms locked looking down on the bar. Then, slowly lower yourself down into the position you would be in at the top of the pull up. It's the same concept that is used for training to do pull ups by doing them in reverse to start.
Another thing to do is get a stool or something stable and high enough that when you stand on it, you are just over the height of the top of your pull up. From here, you will practice the transition from the top of your pull up to the muscle up part. The idea here is that at first, you will be pushing off with your feet a bit. Gradually, you will want to use more and more of your arms until you no longer have to use your feet (think about how an assisted pull up machine works, same concept).
Start each session by trying to do a muscle up from beginning to end. Eventually, after doing these exercises enough, you will start your session with a full muscle up. And it will be awesome.