It'll take a while. One of my favorite quotes is that "perfect posture can exist in every situation or position." Something like that. The same way how if you're on top and your head leans forward, if you're on bottom and your chin starts to drop, you're open to all sorts of head hunting attacks like guillotines, darces, and anacondas. Marcelo Garcia is a master at getting on top and snapping onto guillotines anytime that someone's posture breaks even just a little bit. Just work some of the stuff said in here, and you'll start to naturally keep your head out of risky spots.