It's really just learning how to keep the muscles tight. It's similar to keeping your biceps and triceps tense at the same time when flexing and extending your elbows, regardless of how much weight you're using. You just have to learn to do this with all of your back, abs and hips - be able to flex the muscles when you want which is throughout the rep. It's easier as your muscles get bigger and stronger and as you develop a better sense of awareness over time. You might start off only being able to keep a few areas tight throughout the rep but all you can do is keep learning to get more muscles involved over time. Light work can help in learning how muscles feel if you go for the pump. Eventually you'll know how it feels to be in good position.
For breathing, learn to draw in as much air as possible by expanding your rib cage as you inhale. You can practice by doing drills where you fully exhale for 5 seconds and fully inhales for 5 seconds, then brace your abs and entire back hard for 5 seconds. You might get light headed from doing this so take it easy the first time.