Curl a dumbbell up with your palm facing up, the same as a standard curl. At the top, rotate your wrist so your palm faces down, then lower the weight in that pronated grip instead of returning it the way it came up. That switch is the entire point of the exercise — your biceps do the lifting on the way up, and your forearm takes over resisting the weight on the way down. Lower slowly in the palm-down position; rushing this half defeats the purpose of choosing this curl over a standard one. Rotate back to palm-up only once your arm is fully extended, ready to start the next rep from the bottom.