Zen_Hydra explained specific techniques for hitting through a guard and is realy thorough. I agree that going into any more technical depth would be wrong and might be overwhelming.
Nevertheless I have to give some advice additionally as I could not comment on the original question.
The sad truth is that there is no real strategie on how to punch someone.
You can make one up for yourself but you never be assured that it will work though.
You have to realize that the answer you are seeking for is - to do sparring! Whether that is with a friend or in the gym, the essence is that you learn to land ANY punch properly on a moving target from different angles! You will understand that perfecting punching a heavy bag is something very different compared to a moving target.
After you are comfortable with the movements and dynamics during sparring and can land some hard, precise (what is more important) hits, no matter in what position you are, you should start to implement the stuff Zen_Hydra explained...
... tactics or trying to stick to a plan that you made up for the sparring or a fight as now you don't have to think about the punching itself anymore and it feels naturally for you.
If you consider doing some competitions, sparring is an essential and unavoidable training tool. If you are training martial arts for the purpose of fun, self defense etc. you should include some sparring from time to time but I would focus more on conditioning and endurance... as it is healthier and puts you in advantage compared to a 'standard' street browler.