Pronate your wrists into a shoulder-width reverse (overhand) grip.
Keeping your elbows by your sides, exhale as you curl the dumbbells toward your shoulders until your forearms are nearly vertical.
Hold for a count of two and squeeze your biceps.
Inhale as you return the dumbbells to the starting position.
Comments and tips
Keep your body still and your upper arms by your sides.
Keep your wrists straight.
Maintain a shoulder-width reverse grip.
Instead of curling both arms at the same time, you can, of course, alternate your arms.
Using the reverse grip puts your biceps brachii into a mechanically disadvantaged position, which forces your other forearm flexors (i.e. your brachialis and brachioradialis, especially the latter) to work harder.