You don't need to do anything special, they will just show up if they are in a format recognized by Mac OS X.
So maybe it would be a worthwhile activity to tell a bit more about these external drives.
I am a new Mac user who converted from a PC. I recently purchased an iMac. I have two external hard drives connected via USB to the back of my iMac. I don't see either of them listed when I open Finder. How do I get them to show up?