Internalize the beat and rhythm by listening to the music. Cumbia has a distinct rhythm and a 4/4 beat. Getting the beat in your head will help your feet move in time on the dance floor.
Pivot with your left foot while taking a step back with the right.
Step in place with your right foot while staying on the beat. Count four and bring your two feet back together again. This is the "basic" or "neutral" position.
Repeat the move again but switch feet.
Swing your hips and torso left to right, counting out the beat in your head. Swing your arms in this fashion, too, but keep them close to your side.
Take cumbia dance classes to improve your technique and learn more advanced moves.