I'm pretty sure this is the link I followed and used when I first moved my iTunes library to my external.
Tips:
1) I know you know this, but BACK IT UP FIRST. On DVDs.
2) After it's moved to your external: If you ever unplug your external, and then open iTunes before you plug it back in, go to "Music", then the iTunes Folder, and then delete the folder labeled "iTunes Music" (or relabel it as something else); otherwise iTunes will get confused and won't know where your music is, because you'll have a folder on your HD and your EHD with the same name.
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Tips:
1) I know you know this, but BACK IT UP FIRST. On DVDs.
2) After it's moved to your external: If you ever unplug your external, and then open iTunes before you plug it back in, go to "Music", then the iTunes Folder, and then delete the folder labeled "iTunes Music" (or relabel it as something else); otherwise iTunes will get confused and won't know where your music is, because you'll have a folder on your HD and your EHD with the same name.