Okay I will try:
I use Gimp, it's similar to the Photoshop programm but it's free and you can download it here:
As the programm is installed you have to make a new pic size 800x600 or 1024x768 that's the wallpaper size.
To make your fanart unique you can use brushes, you can find tons of them at this site:
Save them on your PC open them with WinRar and then copy and paste them into your Gimp Brushs folder.
It's also nice to use several fonts, you can find them here:
And here are some site's that show you how gimp works and you can also find some tutorials there:
Just search on google for gimp, there's a lot....
Here's on of my last wallpapers I made with gimp:
Hope this helps...... :)