Assuming your partner is punching you with their left hand, the general concept here is to swim for inside control with your right arm, creating a shield out of your arm. As your partner is throwing this punch, pull your knees in close to your chest, bringing your partner forward much faster than they originally anticipated. While you still have this inside control with your right arm, push their head over to the side, redirecting any potential inadvertent (or intentional) headbutts. Be careful when doing this in practice, as the headbutt is about as likely as the punch to land! From here, you have an overhook of sorts with your right arm, snaking around your partner's arm (either all the way, as with a traditional overhook, or just by pinching your armpit over your partner's arm). Now it's difficult for your partner to posture back up to hit you. Finally, if they try to hit you with their right arm, you can step on their hip, using your shin as a shield (and you can also set up a very nice overhook triangle from here).