You don't "shoot" it, as in something that is done quickly. You should strive for establishing control over your opponent's posture and arms, step by step. Also, don't grab one arm with both hands. One hand is enough, the other should control the opponent's posture by gripping the sleeve or the neck.
My latest revelation for getting the armbar from closed guard is to constantly work to getting one of the opponent's arms across my body. This allows me to control posture using my legs only, pinning the arm between our bodies. Often s/he will notice when I start to bring the arm across and defend, but that usually means that the other arm is open instead. By contantly switching sides, I will sooner or later get enough control to start working the submission.