Don't rotate the leg. Keep the foot in a relaxes position, not pointed, not flexed. Do one leg at a time. Your foot stays in a relaxed position. A lunge is actually a one leg squat.
Shoulder stays over the hips.
Always load the front lunge instead of the back leg or both. Use the back leg for stability. Use an incline bench. While doing a backward lunge, the front foot is the one that is still loaded. The spine is in neutral position.