i crack my elbow by putting my arm fully extended in front of me, palm up (elbow pointing down). i try to straighten my arm as much as possible, actually hyper-extending the elbow joint a little (i.e., opposite the natural bend)
if it's ready to be cracked, i can feel the air bubble give a little resistance.. i just straighten/extend the elbow a bit more and get some cracks.
hope that helps!