The first step is to discover the sensation of a flat back, of firing your lower back muscles, of arching your back, and of keeping or losing your back arch while the hips move. Practice "supermans" on the floor, lifting your legs and chest off the ground. Get into what you think is proper position, then feel with your hand or look in a mirror.
Once you've achieved sensation of proper back position, it's important to get occasional feedback on how your proprioception is doing. I'll ask a knowledgeable lifter to take a look at my deadlift (or clean or squat) and let me know how my back arch looks. I'll also set up a camera with a tripod and see how it looks to me.