Grab a dumbbell with your left arm and place the upper arm on top of the preacher bench or an incline bench. The dumbbell should be held in such a way so that it is at shoulder height. Slowly lower the dumbbell until the upper arm is fully extended, and then use the biceps to curl the dumbbell upwards until your bicep is fully contracted and at shoulder height once again. Squeeze the bicep while in this peak-contracted position before lowering for the next repetition. Repeat for the desired number of repetitions. Then, switch arms to perform the exercise with your right arm.