1. Have your crosshair in vtf format. You can find out how to do that if you don't already with a quick google.

2. Go to your ToonHUD theme, find the crosshairs you have equipped, and count them from left to right (the option to have no crosshair doesn't count)
Equip any crosshair you want, but make a mental note of which crosshair it is. If you want, I can toss in a chart of which crosshairs are which.

3. Once you pick your crosshair, go to the downloaded ToonHUD file and go to this file address:
toonhud/materials/vgui/replay/thumbnails/xhairs

Find the crosshairs you have on your ToonHUD theme by using the above method of identification. The equipped crosshairs should be replaced with your custom crosshair, which should be titled the same as the initial crosshair. For example, if I am replacing the file xhair28.vtf, name your custom crosshair vtf file the same thing: xhair28.vtf.

Once you've replaced the files, you're set! Launch TF2 and enjoy your custom crosshair on sv_pure servers!