Taking a good powerful accurate slap shot is easy, it simply takes a lot of practice. It's not so much about strength but about good technique.
Step 1: Wind Up
When beginning your wind up, you should be positioned sideways to the net with your feet about shoulder width apart with your knees slightly bent ready to lean into your shot. Where you position the puck is up to you but generally you want the puck two feet in front of you. When winding up your stick you can lift it as high as you need as long as it stays in line with your body, do not let the stick swing behind your head. Your bottom hand should be somewhere in the middle of your stick.
Step 2: Shot
When beginning the shooting motion, you must be synchronized in all these steps. As you bring your stick to the ice you are going to begin to twist you hips forward towards the net and shift your weight to your front foot. Again keep your stick in a straight line from you to the net as you bring it towards the ice. If you directly hit the puck your shot will be weak and not too accurate. You need to flex or load the stick so it can whip the puck towards the net. In order to do so, have the stick hit the ice roughly 3 inches before the puck. The puck shot hit slightly before the half way point of the blade.
Step 3: Follow Through
Finally when you have delivered your shot, an easy way to add accuracy too you're shot is to simply point your blade where you want you're shot to go. If your a defense you want your shots to stay generally low so follow through low. Forwards can do what they want.
Step 4: Conclusion
Don't expect to be able to rip 80 mph slap shots in a few days, this takes thousands of repetitions to become good at it.