Step 0
You can't use this method without enabling the developer console, unfortunately. You can do this step by step:

Step 0a:
In main menu, go to Options.

Step 0b:
When you're in Keyboard tab, press Advanced.

Step 0c:
Make sure you've checked "Enable developer console".

Step 0d:
Press OK, then apply, then OK again, or whatever. Just don't press cancel.

Step 1:
Join some random server (we can't do this in menu) and press ~ (the button under ESC and above TAB) to enable the console.
We will do everything there.

Step 2:
Type "sensitivity" (WITHOUT quotation marks " "). There should appear a red text saying "sensitivity" = "8.000000" or something like that. We'll use that later.

Step 3:
Now it's the moment to start using this method. First, choose a useless button that you can quickly press (button that improves our aiming should be pressed quickly). I'll guide you with using button G.
Determine your default sensitivity on which you're playing on. (it's the first number after "sensitivity", so in my example it's:
"sensitivity" = "8.000000" (8).

Step 4:
Binding. Type: bind G "incrementvar sensitivity 4 8 4" Don't worry if you don't understand it, I'll explain!

Incrementvar sensitivity means that one button will toggle the sensitivity. First number (4) is the halved default sens, we'll use this when we're sniping, aiming, or something. Second one is our default sens, so we can toggle between precise aiming and default with one button. The third one is difference between first number and second number. You can experiment with it however you want (change the letter, numbers, anything). I just found these settings perfect for myself.

Step 5:
Now, every time you press G (or the button you've chosen) you switch between good aiming and unprecise (default) sensitivity. You can get amazing effects with this. Good luck! Now you can get on some server and kill everyone using this method.

Thanks for reading.