Position the barbell on your neck/shoulders. Take an overhand grip of the bar. With your toes pointing out and feet shoulder width apart, bring your torso forward and down. Move your hips back to keep balance. Inhale on the way down. Exhale on the way up.
Think of the good morning exercise as a half squat. Your form and preparation for the exercises is identical except that in performing the Good Morning exercise you don't actually squat.
To do this exercise you will first need to position the barbell on your shoulders. There are two ways to hold the barbell. You could either hold it above your shoulders on your trapezius or a little lower, on your rear shoulders. Note that since you'll be leaning forward, the lower along your back you place the weight the easier it will be to perform the exercise. You should place the bar where it's most comfortable for you. Also, don't forget to consider how you grip the bar. You could either have an overhand grip with your thumb on the same side as your fingers, or your thumb opposing your fingers. If you are just starting out we recommend that you take a full grip of the bar, with your thumb opposing your fingers. This will help you balance the weight and prevent it from slipping out. Generally though, especially as you go heavier, the thumb should go onto the same side as your fingers. This will help you keep your wrist and forearms straight, eliminating unneccessary strain.
Holding this grip is acceptable because your shoulders and your trapezius support the weight. The overhand grip just helps to hold it steady in place. If you are performing the exercise correctly, placing the thumb opposite your fingers will not offer any additional support. Keep your elbows back. This will help prevent rounding off the back. Once you have a good grip and position the weight correctly, with your feet shoulder width and toes pointing slightly out, push up and take the bar of the rack. Take one or two steps back. Again position your feet at shoulder width with toes pointing slightly out. One thing to note here, the feet don't necessarily have to point out. You should point your toes in the same direction you would walking.
In performing the exercise you should keep your chest out, your back straight, feet perfectly flat, and hold your head steady. I'd like to once again note that your feet should be flat on the floor and the weight should be evenly distributed across them.
Now lean forward. As you bring your shoulders forward and down you will need to shift your hips back to maintain balance. Depending on your flexibility and preferences you could either keep your legs straight the entire time or bend them slightly as you lower your torso. Just keep in mind that this isn't a squat, you should only bend your knees slightly, and move your hips back, not down. I personally find that bending my knees allows me to get a better stretch and bring the weight down lower, getting a fuller range of motion. If you bend your knees in performing the Good Morning exercise, then straightening them before bringing your torso up will allow you to feel a better contraction. Note that while this exercise targets your lower back, its primary foci are your glutes and hamstrings. The reason your hamstrings work so hard is because they are responsible for pulling your pelvis back into position. That said, don't just bend forward, rotate your pelvis forward (no extra effort involved here, if you keep your back straight, your pelvis will have no other choice but to rotate with your torso).
Come up in a controlled manner. If you push up quickly you may lock out your knees. Exhale at the top. Inhale on the way down.