The guide over on ETF2L is a little outdated and some of the links are dead, so I'll rewrite a more concise one here.
This crosshair is achieved by using a custom font file called "tf2cross" which has the letter "x" replaced with a replica of the in-game crosshair. You need a copy of the font file (tf2cross) which is available on my ftp here:
http://users.on.net/~seeeker/tf2cross/
This needs to be placed in /tf/resource. I chose to install the ttf file too.
The pastie links below need to be placed in your ClientScheme.res in the Fonts and Custom Fonts section (I'm assuming you already have a custom hud and this file exists). Your choice if you want to use the outline or not - it looks smoother without.
http://pastie.org/private/s3qyhdrilbnnwlgqwqfg
This goes into HudLayout.res.
http://pastie.org/private/oo0mx4cuqquqeqe3ont8za
The above two links are optimised for 1920x1080 - if you toggle the default crosshair on and off (as scout) you won't be able to notice it, as the custom tf2cross overlaps the stock crosshair completely. Its important to modify the 'tall' value in ClientScheme.res as well as the x/ypos in HudLayout.res if you want to get its positioning perfect.
Here is an image with outline on
http://users.on.net/~seeeker/tf2cross/guide_images/tf2cross_border.jpg
Here is an image with outline off (had dx8 by accident sry)
http://users.on.net/~seeeker/tf2cross/guide_images/tf2cross_noborder.jpg
Let me know if you need any more help with it, I can probably give you perfect values to use that'll replicate the standard crosshair depending on your resolution.