watch demos like Rhettro said
use low sensitivity
focus on your movement in matches. A lot of times you're just rushing your aim and overstrafing when you can instead just stand still or strafe outside of them a little for really easy shots.
Also, I put an emphasis on not rushing your aim. You shouldn't be flicking immediately to your every target. Yes, there are times when you just have to flick in order to get a quick shot off, but a lot of times you can track your crosshair onto your target at a slower speed, rather than flicking immediately.
Again, it's good to keep your crosshair at the center of your screen at rest, aim and shoot your target, then bring your crosshair back to the center of the screen when you're done. This will become a normal habit once you practice on aimbooster.com enough.
hold your mouse lightly
I don't recommend tr_newbots. The scout bots only strafe sideways. Most of your 1v1s and engages don't look like this in real matches. Usually it's all about prior positioning, or whichever scout is able to strafe outside of the other first to land the easy meatshot on their side (https://youtu.be/3mcto2UsZGA?t=21s). The side of a player model is the easiest place to land a good meatshot when you're up close.
warm up with aimbooster.com. I recommend starting off with training>doubleshot and then with challenge for like 10 minutes or so.
stop flinching when you get shot at. Be aware of the fact that, most of the time, whether you're dueling a scout, or at a mid fight, you're going to get hit. It's normal, it's always going to happen, don't freak out about it.
Personally, I like to just pretend that I'm invincible; that immediately gets rid of my nerves and prevents me from playing like a scared little bitch.
Before I realized the importance of not flinching when I get shot at, my 1v1s were quite inconsistent. Specifically, I would usually be able to hit my first couple of shots, but as soon as I take a 75 dmg meatshot, for example, i'd freak out and start flicking desperately, which would result in my death.
Edit: added more info
Edit2: shoutout to whoever downvoted me