Ok guys, so im just starting off with a little note:
YOU CANNOT GET BANNED. THIS IS 100% UNDETECTED FOR EVER!
Anyway, lets move on.
Step 1:
Acquire a "Sharpie"
Step 2:
I have a bunch of Sharpie permanent markers on my desk for writing on CD surfaces. I took a Sharpie from my desk, put the sniper scope on the screen, and marked the center of the glass CRT screen with a dot (see picture below).
Step 3:
When you mark your crosshair, be sure to look at the screen surface as a perpendicular plane. The monitor glass has a thickness that must be accounted for. The glass is quite thick on my Dell Trinitron monitor, so if you don't look at the screen in direct perpendicular, the mark would be off.
Now, even if you mark the center of the screen at perpendicular, it's most likely that your normal seating position doesn't put BOTH of your eyes at the center of the screen. So the perpendicular center is going to be slightly off. I generally sit slight above and to the left of the screen center when I play FPS games. So I also marked the crosshair from where I normally sit (see picture below). Generally, the actual crosshair is between these two points as I shift around on my chair.
Step 4:
Own at some Hardcore TDM in BO2
Extra:
Erasing the Mark
The Sharpie marker ink is permanent so you can't remove it from the glass even if you rub it with your fingers. This makes the crosshair robust and maintenance free for years to come. But what if you really want to remove it?
The Sharpie permanent marker ink really isn't permanent at all on smooth surfaces. The ink dries on the surface, rather than seeping in. You can easily remove the marking with rubbing alcohol. Take a piece of cotton ball or paper towel, dab a little bit of rubbing alcohol, and watch the permanent ink vanish magically.
This tutorial was made in the event that many people are getting banned using external crosshairs.