Do you flap your shoulders out all the way? Do you put them to your sides? How far do you grip the bar? How far do you go down, 1cm above the chest, do you touch your chest? How far do you go up, do I lock my elbows or do I not. Can I arch my back, is it good to arch my back? What about my chest, do I stick that out? Even the professionals all seem to have a different perspective on it, one may say lock it gives you the full range of motion others may say don't lock especially with heavy weights because you are more prone to injury. Back to the point, I want to clear the air with this one.
So let's try to clear this up, let's get right into it. First lets talk about the positioning of a bench press, you want to lay on the bench flat, back must be touching the bench, the reason why is because incorporating the back easily leads to bad form. Why? Because by using the back you are engaging more muscles that are helping push the weight, meaning you're letting your body compensate for the lack of muscle to move the weight. In short you take less work off your chest to transfer it to your back area. This means you are likely going to have a hard time pushing up in weight because you are not stimulating the chest with enough weight because every time you introduce it to enough weight to stimulate it, the back helps out and takes off the weight your chest cannot push rather than letting the chest the prime mover in this exercise do all the work. With that said everything must be stationary so you avoid using it, notice how I say stationary because you may put your feet up on the bench or in the air to increase the intensity and ensure you are engaging as much of the chest as possible.
How about Hand and Arm Positioning. Your arms must first of all be in a natural relaxed state to maximize the use of the chest and to make the lifting easier so you are actually using you're full potential. If you are not in that natural position and there is strain on your joints and more engagement of weaker muscles you ultimately will have a harder time lifting because you are not lifting efficiently as well as the fact that work is taken off of the chest and distributed more to other supporting muscles. Think of it like a pushing a wall only with your knuckles instead of your palms. If you can close your eyes and picture this you can do the same thing on the bench to help get the positioning right. The positioning hand wise is not where the strips are on the bar, in fact some older bars don't even have those, the positioning is wherever your hands end up so that by the end of the lift you have your hands above the outermost part of the chest. This will not look like your arms are positioned shoulder width this will usually be slightly wider where you almost look like your pushing up and away from the body. Now this next part is important, your shoulders can make your arms flap out so they make a straight line or they can be pressed against the body, you want them 45 degrees right in between both of those. This way you don't put too much stress on the shoulders and you engage as much of the chest as possible.
So now how about locking? With light weight where you can push out 15 or more feel free to lock. However when you start getting stronger or strength training with heavier weight do not lock. The first reason is because you don't want to put that much weight on your elbow joints, it can lead to injury. But let's pretend you don't care about that and you feel you know how to lift and you lift safe, by not locking your giving yourself a strength gaining benefit of keeping the muscle engaged for longer and is actually highly recommended to build muscle which can be done with any exercise.
How far are you supposed to go down? The answer to this is not specific because we are all different and also have differently worked chests, some bigger some smaller. Saying to go 1 inch away from the chest is ideal is like saying curling 25 pound dumbbells is ideal. It just doesn't apply to everyone the same. Realistically you want to go as far down as it takes to get your arms going 90 degrees. If you drop your arms past that point you are only stretching your chest and tensing your back. Keep in mind you are not working the back simply because you are lying upright with your chest to the air and back to the ground meaning your chest is not resisting any weight and there is no gravity acting on it so it is not being worked. 90 degrees is as far as you need to go, past that is pointless.