Use the Create a server option on the opening screen for CSS and choose any map and start OR while on the
TG server:
Go to console (the ~ button)
You may have to enable it. To do this, Click Options, Keyboard, Advanced, and click Enable developer console.
Once your console is OPEN, type in "status" with no quotes. Look for the players current name, and the number that begin with STEAM_ is the steam ID. Get that whole number. A layout is like this: STEAM_0:1:9999999
It can be any length in numbers but make sure you include the STEAM_ part in your profile and when providing the info to admins.
You can also do this while in any public server.
