For bicep curls, a weight works best but you can use anything that adds some resistance.
Sit on a chair with your legs slightly spread in front of you and feet flat on the floor. Bring your torso forward slightly and keep your abdomen strong. Hold the weight in your hand, resting your elbow on your thigh, elbow bent and palm facing up. Place your other hand on your other thigh. Lift the weight all the way up without moving your upper arm. Slowly bring the weight back down. Repeat eight to 12 times.