Set-up: Flex your working arm at the elbow and grasp it with your other hand, facing the palm of the working hand towards the floor.
Movement: Contract the bicep and forearm of your working arm, while controlling and resisting the contraction with your other arm. Do not let your upper back round over - consciously pull your shoulder blades together to provide a stable base for your arms.
Emphasis: Keep your core tight and a neutral spine. Focus on the contraction in your forearm and bicep.