Smith machine inverted pull ups:
With your feet on the floor in a wide stance in front of the Smith machine, position your upper body underneath the bar of the Smith machine, holding the bar a little wider than shoulder-width apart with your palms facing down. Hang from the bar with your arms extended fully, and allow only the heels of your feet to touch the floor. Keep your body straight—don't let your butt sag down). From the starting position, lift your upper body up until your chest touches the bar. Focus on retracting your shoulder blades down and back and opening up your chest in order to maintain proper posture and to activate your lower/middle traps. Reverse the motion and lower yourself down until you are back at the starting position.
Perform the exercise slowly and control your movements.
When your arms become stronger, you will be able to perform classic pull-ups.