This is the bread and butter of Brazilian Jiu Jitsu. Everyone loves the basic armbar from mount but very few people look at the finer details of how to train it as a white belt.
In this video I break down the beginners version and then show the full speed version. Keep in mind that you need to be careful not to knee your partner in the face and make sure to swing the leg over the head BEFORE you fall. If you stick to doing it the way that I show in the video you'll have a lot of success!