With the gun closed, hammer down and empty, hold the gun firmly and slowly pull the hammer back for single action.
Okay, I will go ahead and post it anyway. I was going to ask how slow because I have one that if I go very slow it doesn't go, BUT... I realized it was just a matter of not actually pulling the hammer back far enough because I hear the trigger fall into position before the end of the full hammer retraction. Now that I know about that; it locks every time, even when I grab the cylinder while cocking it.