1. Download the spray you want to use.
2. Browse to C:/Program Files/Steam/steamapps/<your_steam_ID>/team fortress 2/tf/materials/VGUI/logos/
Note: <your_steam_ID> is whatever your steam login is.
If you can't find the folders 'VGUI' or 'logos' in the 'materials' folder, simply create them. Make sure the 'logos' folder is inside the 'VGUI' folder.
3. Put the .vtf, .jpg, or other supported files for the spray into the folder. That's it.
1. Launch Team Fortress 2.
2. Click Options > Multiplayer and select the desired spray from the drop down list.
Note: If you don't see the spray in the drop down then double check that the spray files were put into the correct folder.
3. Join a server, walk up to a wall and press 't'.
Note: If you change the spray mid-game, you have to wait until the server changes maps for the new spray to take affect. You could also disconnect and log back in.