It’s always great to review a basic armbar from the mount. In this video, Firas from the Tristar Gym shows some great details in securing the arm. He makes sure his chin is tucked in and that he is putting a lot of pressure with his thigh on the opponents body. Then he moves into S mount, and secures the arm.
Failed Arm Triangle to the Mount. If you are having a tough time submitting with an arm triangle, you can transition to a triangle from the mount. Taught by Scott Epstein.