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.
Go to program\steam\steamapps\username\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.
Copy the vtf and vmt files and paste them into the logos folder.
After that, create a new folder in logos and name it UI.
Copy only the vmt file and paste it in the UI folder.
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.
Join a server and spray on a wall (by pressing T), it will work if you've done everything correct!