For this you will need a Partner or a mirror to practice.
Step 1: Getting in the Orthodox Stance
The orthodox is where you're leading with your left foot and your right foot is in the back.
Step 2: Put Up Your Guard
Lets go with the textbook guard. Place both hands (already in fist) near your chin, with your chin slightly tucked in.
Step 3: Know Where to Aim
The heart is what you are aiming at. It will be on the left side of there torso, close to the middle of the chest.
Step 4: Make Sure You Have a Good Chance of Hitting
This is more of awareness. Make sure, if you are using a partner, that they are not blocking, as punching an elbow can lead to a broken hand.
Step 5: Strike!!
Punch your target or the reflection where the heart would be with a right straight. Do not actually punch the mirror