Learning to perform a Kimura takes time and practice.
Kimura can be done from multiple positions, but is most often performed on fighters trapped in a full guard.
See the picture, Kimura Vs. Gracie in his winning armlock.
Step 1: Control the Opponent's Arm
Step 2: Grab Wrist Firmly, Release Leg, & Sit Up
Example: If you were on the left side of their body, this would be your right hand on their left wrist.
Step 3: Grab Your Own Wrist
(Hint: Make sure to maintain control of their body while performing this move.)
Step 4: Important: Maintain Control
Make sure the trapped arm and elbow are pressed tightly against your chest, keeping control. If you cannot keep control, you are at risk of losing the submission as well as a reversal.
Step 5: Simply Twist Your Body
Step 6: Tapping Out
The final step is to do this altogether in one swift motion, making your opponent tap out.