As you are curling the barbells there should be a slight arcing motion with the upper arms coming forward. (ptinto.com)
The way you set the grip width is by inserting the quick - pins into the desired hole on the bracket (i.e. one of the bottom 4 holes that are in the slight arc; see image above). (kingofthegym.com)
After touching your chest, press the bar up and back towards the starting position in a slight arc and think about bending the bar with your hands. (coachcalorie.com)