So this is clear, you want you iTunes media on the external drive? Why not put the whole library there? It makes it a lot easier to move in the furutre and if the external drive will have to be on in order to play anything then you might as well keep it all together neat and tidy.
You don't say how you had it organized previosly which makes it hard to tell you what needs moving where. If it was all together before, copy the entire iTunes folder to the desired drive, then start iTunes while holding down the option/alt key and guide it to the iTunes Library.itl file on the new drive.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
Where are my iTunes files located? - http://support.apple.com/kb/ht1391
iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847