Pull it off while training here and there, people rarely see it coming! The hardest part is getting that leg across their hips quick enough. Learned it from an Ian Mcpherson video which I'll post here. He scoops his controlling arm under the guy's head and hooks the shoulder (youtube video picture shows this) instead of going across on top which seems a bit less secure maybe? Learned it as a white belt and use it, the paper cutter, or an armbar depending on what they are defending and how. Pretty nasty combo!