Similar to correctly performing lots of movements, it's about where you feel tension. With biceps, you should feel tension in the front of your upper arm without relying on much momentum or swinging.
Likewise, your elbows should stay relatively still, and your grip should feel firm but not painful or strained. Smooth reps can also indicate a good level of control. If your shoulders or lower back begin taking over, try reducing your range of motion or your total load, and restart the movement.