Start from a dead hang with a grip wider than shoulder-width with and the arms fully extended.
Initiate the pull by depressing and retracting the scapula (pull shoulders down and back)
Pull your chest toward the bar, leading with the elbows.
Pause briefly at the top when your chin clears the bar, then lower under control to full extension
When you start working on this exercise, don't go too wide directly: go a bit wider than your usual Pullups, then wider the next session, and so on. This will reduce the risk of injury.
Risks
The elbows-out position can put undue stress on the shoulder. Practice this exercise carefully to avoid injury.