iTunes on external hard drive - 2 computers

I have previously set up iTunes on my iMac and keep all my music files on an external hard drive. This works great. I now want to have iTunes on my Macbook Air have access to the music files on the same external hard drive. How do I do this? All I can find is help moving an entire iTunes library from a computer to an external hard drive but my files are already on the external hard drive.