Perform basic choke except you start with your head on the mat. Then slide in your first hand to the collar. If that hand is your right, place head at 1 o' clock. Use free arm to make an arc from your left to your right flank. You can avoid a sweep this way. Slide your free hand and reach the other collar. Make a sharp 12 o' clock turn to finish the sub.
If you op defends by grabbing your forearm, just stick out your elbow. Slide your free arm under the space your elbow created and finish the choke.
There is another technique on passing a half guard. But it's too complicated for me to explain. Maybe next time then.