Select a lighter dumbbell weight with one hand. To get into starting position, hinge at the hip to lean forward with your arm hanging perpendicular to the ground while the unoccupied arm is at your side.
Maintaining a fixed upper arm, curl the weight upward.
Really emphasize using your bicep not added movement or other musculature.
Bring the dumbbell to the top and pause briefly then lower to the bottom slowly.
Repeat for desired reps. Switch to opposite arm and repeat exercise.
Heavy weight is not necessary for an isolation movement like this
If necessary, you can balance yourself by holding onto something with the non-working arm.
The muscle used first will be used the most. Momentum will negate the movement.