You must begin with your opponent in your closed guard
. Once you have your opponent in your full closed guard pick which arm you want to go after. For purposes of instruction lets just say you decide to go after his right arm. So start out by securing his right arm as much as possible by grabbing a hold of his tricep with your right arm and with your left wrist getting a good solid grip of the end of his Bjj Gi sleeve.
Next, let go of your closed guard maintaining control of his arm, place your left leg on his hip. Bring your right leg high across his back and press downwards to break his posture. Then pivot your hips to a 90 degree angle of your opponents body. Make sure to pivot to the right side or the opposite side of the arm you are trying to arm bar to get optimal leverage. Keep in mind to pull this move off during training you must be able to do all three of these actions in one fluid movement. This will only be achieved by practice and muscle memory.
Lastly, pass your left leg over your opponents head to get in the position of the arm bar to finish the submission. To make him tap out simply squeeze your knees together, raise your hips and pull down on his secured left arm. This will surely finish the submission and make your opponent tap out. If you are a beginner be sure to practice this move with a Bjj Gi because it will help you a lot with your technique which is the most important aspect of Brazilian Jiu Jitsu. Without the Bjj Gi your opportunity for growth and mastering your submissions will greatly diminish.