1. First of all, download a spraypack in .rar format here at GameBanana.com. Extract it with WinRAR (see screenshot) and go into it. It should contain a vtf file and a vmt file.
2. If it contains a folder called materials or VGUI, Copy materials or VGUI and replace it instead of materials or VGUI in the tf folder. Windows will ask you to replace files, click "Replace all", go to point 6.
3. Otherwise, go to program\steam\steamapps\common\team fortress 2\tf\materials and create a new folder. Name it VGUI and go into it. In the VGUI folder, create a new folder called logos if one doesn't already exist.
4. Copy the vtf and vmt files and paste them into the logos folder.
5. Copy only the vmt file and paste it in the UI folder.
6. Launch Team Fortress 2 and click Options. Go to Multiplayer and chose the spray from the list (see screenshot). If it's not in the list, then something is wrong.
7. Join a server and spray on a wall (by pressing T), it will work if you've done everything correct!