Barbell Flat Bench Press
When doing a bench press, position is key. I would see a lot of people struggle with form and round their shoulders forward, lift their butt up, or whatever.
Grip the bar tight with your hands, about shoulder width (as if doing a pushup). Be sure to keep your hips and butt on the bench and drive your heels into the floor as if doing a glute/ham bridge (your glutes should be activated). Arch your back to tighten the lats and secure your shoulders into the bench. As you lower the bar to your chest, act as if you're "pulling" the bar into your chest with your back muscles and using your chest to descend slowly. As you touch your chest, press the bar back upwards while keeping your back tight still and finish over your shoulders.
Also, try not to flare your elbows out too much. Keep them in tighter to your body, about 75 degrees away from your core). Having them almost perpendicular to your body can cause a shoulder impingement or other issues. If you feel any pain in your shoulders, consult with your trainer or physical therapist.