If you want a strong punch, it's really about a lot of dedication. As long as you have the basic technique right, you only need to keep on punching as much as possible. For a beginner, you could just use a punching bag. Obviously, punch barehanded since you won't have gloves in a fight. Start with weak ones so you avoid accidentally injuring yourself.
Ever so often, escalate the intensity. Keep on punching the bag and keep a steady tempo. Make ever punch count and put in maximum effort with every single punch. Make sure to twist your whole body whilst punching in order to add more momentum and power. Your back foot should definitely be turning.
The hardest part will actually be the thirst. Even the skin on your knuckles ripping can't quite compare to the thirst you'll have.
My method is basically to train one punch to perfection as that's all you'll hopefully ever need.
Everyone has a plan until they get punched in the face. That's why you need to keep your plan as simple as possible. If confrontation is unavoidable, go for the dirty surprise punch and keep on punching. Otherwise, just run.