These two pre-swing fundamentals are sometime confused and taken to mean the same thing. They refer to different things, and must be done correctly to put your body in the best position to put your best swing on the ball.
Stance is where your feet are both in relation to the ball and your target line. Posture is the shape of your body as you address the ball. The following description is for a full swing.
To get into your stance, take your grip with your right hand. Put the clubface behind the ball, aimed at the target. Step into your stance so your feet are parallel to your target line, and put your left hand on the grip.
Your feet should be about 18 inches apart, but this depends on the club you’re using. With a driver, your feet will be a few inches farther apart, with a 9-iron, a few inches less.
The ball should be in the center of your stance for any shot hit off the ground or irons off a tee. Put the ball two ball-widths forward of that to hit your driver or fairway metal off a tee.
Stand up straight. Now push your hip straight back, keeping your head where it is. This will cause you to bend over at the hip, while not rounding your back or drooping your head. Even though you’re bent, you should still have the feeling of standing tall.
Let your knees bend slightly, too. Your weight should be evenly distributed from the front to back of the soles of your feet.
Look at your elbow as your arm hang downs at your side. Notice there is a slight natural bend in it. This is the full extension of your arm. If you straighten your arm, it’s now over-extended and tense. If there’s more bend than normal in your elbow, your arms are under-extended. Your swing movements will be constricted and weak. Hold the club with this natural bend in the elbows.
With shorter clubs, the arms should hang straight down, relaxed, and at their full natural extension. With longer clubs you will stand straighter, so your arms cannot hang straight down, but there should be no feeling of reaching out for the ball.
The shoulders should slant downward a bit from left to right, and the line across them should be parallel to the line across the hips. Do not let the right shoulder come forward so the shoulder line points to the left of the hip line. This is an easy error to make, and one that will make the direction you hit your shots inconsistent and unpredictable.
Your head will fall slightly from its upright position when you bend over, but do not let that make you hunch your shoulders.
Practice your stance and posture at home by leaving a club in a place you pass by frequently, and whenever you pass by, stop, grab the club, and set up. It only takes a few seconds, so there’s no reason why you can’t do it quite a few times a day. Every time you set up, not just going through the motions, but paying attention to every detail, you’re that much closer to having a good setup become a habit.
You might think if the stance you have now is comfortable, it’s right. Comfortable only means habitual. Practice the right stance until it becomes comfortable.
Check yourself in front of a mirror. A stand-up mirror costs about as much as a round of golf and is an excellent investment in your game.