Preparation
Sit on curl machine placing the back of the arms on pad. Align the elbows at the same pivot point as the fulcrum of the lever. The seat should be adjusted to allow the arm pit to rest near the top of the pad. Grasp lever handles with underhand grip.
Execution
Raise the lever handles until elbows are fully flexed with the back of the upper arm remaining on the pad. Lower the handles until arms are fully extended. Repeat.
http://www.exrx.net/WeightExercises/Brachialis/LVPreacherCurl.html