The layout.nut is just a text file. You can edit it with any text editor (i.e. notepad, write, vi, nano, Notepad++, etc.)
The backgrounds are image files (.png or .jpeg) and can be edited with something like mspaint or Photoshop.
FYI - Don't expect for this to be a simple task. It's not rocket science but it's also not something most beginners will try to do.