Summarize:

You should have no more than three options for shots any time you get up to the box to take a penalty kick in soccer. Don't make it more difficult on yourself than it needs to be. Practice taking three differently placed penalty kicks, and call that your repertoire of shots. Perfect each one of them, so you know you'll be able to score in any of the three places when the time comes and you make that decision. Wherever your most comfortable, highest-probability shots are, practice them and don't worry about any other choices.  Most goalies will dive left or right at random, making it very difficult to anticipate where the goalie will be when you shoot. Since it happens so quickly, you're playing a game of chance, always. If the goalie is familiar with your style of play, however, it's good to have a few go-to options in your bag at all times. Again, more shots are missed than saved, so it's mostly up to you. Most saves happen in the bottom right corner of the goal. Some goalies guess that right-footed kickers will try to outsmart them by going to the non-natural corner. It's best to go easy and uncomplicated the process. Shoot to the corner that feels best. Anyone can blast a ball into the back of the net a couple times, but what about when you've been going hard on the field for half an hour, fighting for balls, taking corner kicks. Your legs will be tired, you'll be sweaty and exhausted and all of a sudden all the eyes will be on you. Your legs might feel like lead, but you've got to come through with a brilliant goal that'll put your team ahead. Train the right way. Take penalty kicks when you're tired and learn to slow down and calm yourself, focusing on the mechanics and the motions to help yourself score. For some players, a two-step approach will be plenty to get the power they're looking for. Others might need a different amount of steps, or may want to play around with psyching out the goalkeeper with different approaches and fancy footwork. That's fine. Practice approaching your kicks from different steps back and see what feels the most comfortable for you. Some players like to take a few more steps back to force a stutter-step, a few little quick strides before driving into the ball. This helps to throw the timing of the goalie off, and can force the keeper to jump early, meaning that you might be able to shoot into an open net. Again, easy to blast balls into an open net. Practice with a keeper who's trash talking. Practice while your little brother makes fun of you from behind, telling you that you're going to miss. Practice when there's loud music playing, bugs howling, and it's raining out. Practice in the worst conditions and you'll be prepared for anything. Time to get zen on the PK game. If you want to really take your penalty kick shooting to the next level, practice them with your eyes closed. Literally. The distance from the penalty spot to the goal and the dimensions of the goal will be identical every time you want to take a penalty kick. This means that your approach, your mechanics, and your shot-placement should be automatic. You should be able to do this with your eyes closed. Why not try it?
Develop a repertoire of shots. Take penalty kicks when you're tired. Measure your approaches and practice different steps. Practice with distractions. Practice blind.