How do you use sprays in Team Fortress 2?

Solution 1:

In order for sprays to work properly, you must:

• Have a spray image. You can download and install a premade one, or create your own by converting an image to VTF format. (I'm not 100% sure if the default checkerboard pattern counts as an actual spray or not, and I can't currently test this)
• Have sprays turned on in the multiplayer options. I believe you must also allow custom content to be downloaded, although again, not 100% on that one.
• Be playing on a server that has sprays enabled. Not all servers do, so if you can't see anyone's sprays, try a different server.
• Press the Spray key (default T) while pointing at a valid target.

If you're doing this right, you should hear a "spraycan" sound effect. Some surfaces don't take sprays properly, although a flat floor or wall is usually safe. You also have to be somewhat close to the surface in order to spray - too far away and you won't be able to. There's also a cooldown between sprays, so you can't spam the spray button.