Sometimes your opponent will keep their elbows tight and block the gi choke. Presuming the overhook hand has the lapel grip, take the hook thats on the overhook arms side out and turn your body in the same direction. Take your near leg & cross it over your opponents stomach with your foot on the opposite hip now. Control your opponents leg by grabbing the pant leg, lean back and extend your arms in opposite direction to apply the Bow & Arrow choke. Another way of securing the submission is taking the leg thats crossing over and instead use that to wrap around your opponents arm, allowing yourself more control of your opponente.
1) Transition to Bow & Arrow Choke
2) Bow & Arrow Choke Finish