You learn the fundamentals of marksmanship, the basics, at the academy, and revisit this training as time goes on during in-service refresher classes so you don't lose what you don't use. If you're dedicated to your training, you also learn to add advanced techniques to your toolbox here and there. And if you push yourself to continue to learn and grow and you practice what you learn, the actual physical mechanics of manipulating your weapon become so ingrained that you don't have to give them much thought.
Tactics
After you become proficient at the physical manipulation of your weapon, you eventually have to put into play the actual use of the item in a practical application. Theory can only get you so far. You can practice the moves necessary to operate a car in a parking lot, but until you get out on the road you don't really appreciate the practical aspects of the vehicle's operation in traffic. Or to offer an example with perhaps the same potential for catastrophic results, would you want a surgeon working on you who attended all the classes but had no actual experience with a scalpel?
The metaphor of the surgeon is very similar to what an untrained officer who doesn't know how to tactically respond might experience deploying his or her weapon at the scene of an incident. If you are that officer, you know how to punch holes in paper on the range, but putting your firearms skills into play when necessary in a way that will keep you alive while moving to cover, working the angles, and keeping innocents out of the line of your fire is another set of skills you must also master.
So you must practice tactics and practice them some more. Train yourself to slice the pie around corners, to work stairways, to move as a team with fellow officers, to use cover, and all of the other tactics that will allow you to use your weapon efficiently in the field. With enough training eventually tactics, too, become second nature. This will allow you to concentrate on making good, legally justifiable decisions.
Decision-Making
The most important aspect of your response to any deadly force incident is the decisions you make.