First, you need to find the iTunes data files! I can only tell you that I backup my itunes library ever 30 days. This is how I do it using a simple file copy in Windows Explorer:
1. Find the iTunes Library (mine is in a folder on the 'C'(main) disc with the label of 'Libraries/music/mymusic/iTunesmedia/Music/' (The deepest folder contains the iTunes data by artist.)
2. I then simply copy the Music folder and subfolders to my external disk with a name of the date&iTunes.
3. After the copy is completed, I delete the second previous backup copy. (That way I always have two full backups.)
Works for me.
You can find the location of your iTunes data using the iTunes software by going to the 'Edit' - 'Preferences' - 'Advanced' window.