Rest the flat side of a single dumbbell on both your hands. It is important to rest the dumbbell plate on your hands, instead of “holding” the dumbbell. Holding the dumbbells will require grip and consequently activate the forearms. The purpose of the exercise is to try and isolate the biceps, so by resting the dumbbell on your hands, you are focusing exclusively on contracting the biceps to lift the dumbbell. Your wrists will stay bent back (so your palms face upwards) during the whole movement to provide a flat surface for the dumbbell to rest on, but also to open up the bicep heads more during the movement.
Start the movement with arms slightly bent, with the dumbbell resting on your hands. Contract your biceps to lift the dumbbell upwards by bending at the elbows. Hold for a moment, before slowly lowering the dumbbell back down.
The dumbbell should not rotate as your palms should remain facing upwards.