Half guard, top
Half Guard to Head and Arm Choke
From the top of half guard, trap one of their arms across their face, free your leg and slide to mount, then lock the head and arm choke and finish it off to the side.
Step by step
Isolate the arm
Get your head down next to theirs and push their near arm across their neck, pinning it there with your head and shoulder.
Free your leg
Keep your chest heavy and work your trapped leg out of their half guard, using your free foot or knee to push their knees down.
Slide to mount
Swing the freed leg over their body and settle in mount, still holding the arm across their neck.
Lock the choke
Wrap your arm around their head and the trapped arm, and grab your own biceps with that hand. Your other hand goes on top of your head.
Finish off to the side
Step off the mount to the side of the trapped arm, drop your hips to the mat and squeeze, driving your shoulder into their neck.