Having just done something similar its a fairly simple and pain-free process (please note caveats in place that should it screw things up its NOT my fault .... guess who works in IT
)
Firstly ensure that iTunes is managing your music and its file locations, I think this is one of the options under preferences. This basically means all of your music is stored in folders in your iTunes directory.
Then simply copy the whole iTunes directory on to your external drive.
Then when you want to start iTunes simply hold the <shift> key down when you click on the icon to start and it should bring you up a box asking for you to Choose Library .. simply click that and navigate to the folder on your external drive that you copied iTunes folder to and select the iTunes.itl file.
This should then load your iTunes library and job is a good 'un.
You may have to authorise all your music for your laptop but again that should be a fairly simple task from the <Store> menu.
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Apologies if the above is a bit mudlded is was just a quick brain dump.