This video is quite good at explaining it. You have to pay attention that the half where the yarn is coming from the ball on the right needle, else it won't join. I already had that problem.
Somehow I get the impression that she has an italian/spanish or romanian accent... Anyway...
This one is good as well. Might help you some as well
Oh I'm sorry, the videos are shown in continental knitting. You might want to search on youtube for "magic loop knitting", or even add "english", if you're an english knitter.
And don't forget to make the first stitch of every "half-round" really tight.
Oh, and the last stitch of every half, when sliding up to the needle again, is really tight, but that's normal. It might be some fight to get it up the needle...
But I really like magic loop circular knitting. I even did something "magic loop double knitting". That was a fun experience as well.