Well im sure you know to bend at the knees and not bend your back. Lower your level and keep your chin tucked. Dont worry about getting kneed in the face, go low.
Im still a white belt and forget to tuck some times myself. But its been forever since I tapped to a guillotine on a TD. 10 minute drills of take downs only really sharpend my foot work and timing.
Going down to the knee and not bending your back is right, but for some reason people still think you should tuck your head down. When you go in for a shot your back should be straight and your head should be straight up like you're trying to look at the ceiling. This makes it much harder for your opponent to get around your neck. If they do get around your neck always dump them to the side and try to immediately pass to side control cross side and work on the arm to pop the head out.
If someone gets around your neck you also have other options. If you come in and your head goes off to your left (his right) and he catches you, come back up to your feet. From here throw your right arm over his left shoulder and throw your head straight up like you are looking at the ceiling, which should relieve some of the pressure from the choke. From this point use your left hand to attack the choking arm. Now you have some options...you can trip, turn out of the choke, etc...
I was taught the same thing about tucking my head when I first started doing BJJ. Later I was taught to shoot with my head up like I was looking to the ceiling. Since changing to shooting head up I have gotten choked a lot less and honestly don't really worry about the guillotine anymore because if the head is up high enough it's really hard for him to get his arm around your neck.