Consolidate multiple iTunes libraries on External HD to use with iTunes Match

I have a backed up iTunes library on an external HD because I rebuilt my macBook Air. I now want to move it to another External HD, consolidate that library with the one I have been building on my laptop for the last 6 months and use iTunes Match for 1 library.

There seem to be multiple ways of copying or building the libraries themselves. I would like to keep playlists but it is not absolutely necessary. Where do I start?