How to bench press:
Lie down on the bench. Grab the bar with a medium grip-width and lift it above you. In relation to the shoulders, the arms must be at right angles. This is the original position. Breathe, and slowly lower the bar to your mid-chest. Pause a little. After that lift the bar while exhaling, return it to the initial position.
Keep an eye on your chest muscles: you should feel tension in them. After each approach, fade for a second, and then lower the bar again. The time for raising and lowering must be equal.
If this exercise is new to you, do it with an assistant. Be careful with weight. Do not let the bar move forward.
You need it to touch only the middle of the chest, and nowhere else.
Do not move the bar off your chest. You have to control it completely. To perform the exercise correctly and protect the shoulders, you must bring the shoulder blades together.
Arching. There is no consensus on arching. Many athletes believe that arching should take place only in powerlifting. But, in fact, arching allows you to reduce the trajectory of lowering the bar, and as a result, allows you to lift heavier weight, since in this position the lower part of the chest muscles is working.
The most serious mistake is "bouncing" of the bar from the chest. In this position, it is almost impossible to activate the muscles; that can lead to fractures of the ribs and stretches of the intercostal muscles. You should bench press smoothly, working with your chest muscles.
Turn the elbows to the sides. This position is fraught with a rupture of the pectoral muscle. Placing the elbows to the sides, the athlete involves the broadest muscles of the back, which is why the effectiveness of the exercise is significantly reduced.
Hips coming off the bench. The proper press technique includes three points: blades, hips and heels. When you tear the hips off the bench - you carry the weight on the legs and blades, while the lower back receives a strong load. In this position, it is not only difficult to keep balance, but you can also hurt your back.
Wrong breathing pattern.

 Bench press. Wide grip.