Something about walking out to a range and putting some holes in paper is just relaxing. If your interested in the art this is the difference between doing clumsy dumb shit while looking cool and being able to put a hole in a 10 inch target at 800 meters.
Principals of marksmanship (Tripods are for pussies edition)
First and most important is bone support. For long range shots you never hold the gun with your muscles. You use your skeleton. This is all geared towards the prone position.
Tie off a 2 point sling on the front of your rifle in whatever way works for you. Tie the other end around the bicep of your non trigger arm and wrap your hand in the sling adjusting the length until its tight and bracing your elbow makes you feel like you are totally attached to your rifle.
The 2nd part of contact with the ground should be your trigger arms elbow
The 3rd part is your body
When you are shooting there should always be a "Natural point of aim". You can test this by having someone walk up and shake the front of your rifle around (on a level that would simulate recoil of a high power shot) Without using any muscles at all this point should always remain a constant. To adjust your aim you should move your entire body behind the rifle. Think of your rifle as the unmovable object and yourself as what moves. For small adjustments move your stomach. For large adjustments move your hips and follow with your stomach. Your legs should always follow your hips to something natural.
Second most important is your trigger pull/breathing. (Some will argue its your sight picture but I call bullshit because if your bone support and trigger pull is good then you will hit consistently and can adjust your shots so long as your sight picture is consistently dicked up)
You do not pull on a trigger. Its a slow steady squeeze at the bottom of your breath when you are between breathing out and breathing in. When you slow your breathing down there will be that point in between that you are naturally doing neither. This is when you barely squeeze. The trick is to not even know when the rifle is going to go off. You line up your shot and with each breath squeeze a tiny bit. The reason for this is that if you yank the trigger back your shot will go to the right. If you know when the shot is going to go off you will anticipate the recoil and instinctively throw your solder into the butt of the rifle and it will go left. You want your hand high on the grip with the base of your finger on the trigger. Your breath out is the lead up to the shot and you should be breathing out of your nose (Kind of flexing that thing that lets you block out breathing out of your nose so you breathe out very slowly) Thats the basics. I could probably write 1000 words on trigger pull/breathing control and what defines a good trigger setup but mastering what I have covered will let you hit targets at 500 meters no problem as long as you learn wind calls.
Third is your sight picture / sight alignment. Your sight picture is you looking down the sights at your target. Your sight alignment is the alignment between your front sight and rear sight. If you are running a scope this doesn't really apply to you but if you need a scope to hit a target 800 meters out you are on par with the pussies that need a tripod or a bench to shoot straight. There is not much to say about this other than be mindful of it and get good at it.
Fourth is your wind call. Its where things get complicated. You have to know your sights and be able to adjust them according to how you read the wind as well as where you impact after your adjustments. A good point of reference is 3-4 shots. If you shoot 3-4 shots and they all hit in the same place but off target you are shooting right but your sights are off. If your shots are all over the place you fucked up something I covered above. Most likely your bone support/breathing/trigger pull. You can use trees, a wind sock or even dirt to judge the wind. Past 700 meters you will need to judge the wind where you are as well as the wind where your target is independently. I am not even going to touch on that but if you are at a nice long distance range there will be berms on either side that reduce this effect and make it negligible on a 6 foot target.
Thats all long distance shooting. Now for tactical shooting.
First and most important is muscle memory. The trick is to be able to look at something and decide to shoot it- there should be a split second between the time you decide to shoot it and the time you do. The way you do this is constant practice. Practice doesn't make perfect but perfect practice does. Start slow and stay slow until it hurts to move slow. Move your head slightly down and to the left to where it would be if you were shooting and bring the rifle up to it. Repeat this for an hour a day for a month and you should be half ass ok at it. The idea is to get to the point that you can look at something and bring your rifle to shoot it without thinking or adjusting your point of aim. When you can look at something, snap the rifle to your eye and hit the target without thinking you have mastered muscle memory. Some people find it helpful to keep their pointer finger forward on their off hand as if pointing at the target.
Second is movement. The simple principal is 'Step' 'Pivot' Present'. If you need to shoot to the left you step forward with your right foot, Pivot to the left, and bring your rifle to your eye. If you are clearing a building do not ever keep your rifle static. Move it in a figure eight style as you clear. Things in motion stay in motion and it can mean the fraction of a second in reaction time that matters. Learn how to turn around in the open vs in a tight situation where you have to bring your rifle down then back up because it it too big to just swing around. Learn how to fall down and shoot behind you or in front of you. Start slow and do it slow until it hurts. Then move fast.
Third is anatomy. Understand where to shoot and how to shoot. The most effective is the double tap. If you shoot twice in the chest regardless of if they are wearing armor that prevents those hits from penetrating the kinetic force of the hits can shut down their organs. The other killing points of impact are to the head and to the pelvis. Crippling points are the shoulder, knee and pelvis. The knee and shoulder are the most painful followed by the pelvis which is more because of the nerve endings in the back (In that order). For head shots you have to imagine an upside down triangle going from between the eyes to the bottom of the nose. A shot there will travel to the back of the brain and shut down any brain function immediately. Contrary to movies you can take a shot through the heart or even 20 shots in a second and keep right on going for a few seconds. Thats more than enough time kill. Its only in the movies you see someone get shot in the chest and die immediately. Pelvic shots are kill shots because they cut through the digestive tract and release all that nasty bacteria and waste into areas that don't want anything to do with it. Its a slow death but one thats hard to stop. This brings us to the most effective technique- Failure. Double shot to the chest with a single shot to the head or pelvis. Practice that over and over.
Fourth is target priority. If there is a guy at 3 meters with a knife and a 2nd guy at 10 meters with a gun you have to make a call at which to engage first. This simply depends on your ability and judgment of that ability but its something you need to focus on and understand. This is where the ability to read people at a glance can save your life and micro expressions are important. The drawn eyebrows or wide eye rage should die first. The cold dead eye and the out of place smirk second. Learning to read people is much more critical if you are on the defense..
Last (and arguably most important depending on your situation) is the group dynamic. If you are working with 1 other person knowing how they operate and how to clear with them. One takes the fatal front/dominant corner and the other pies off. Or working in a 3-5 man team in a formation. Or working in that 3-5 man team in a formation of 3-5 man teams in a coordinated way. This gets really complicated and is why the military is a thing. Its not a good shooter. Its a group of good shooters that know how to work together as a team with excellence in the basics.
Above all. Slow is smooth. Smooth is fast. Try to go fast in your first day week or month and you get sloppy bullshit. Practice prefects the level you preform so perfect practice makes perfect.