Three big steps. 1. get the drive ready while directly connected to your MAC. 2. Get the physical connections made. 3. Configure the drive on Airport Utility and access it in FINDER.
1. I like to directly connect the drive using a USB wire, and use DISK UTILITY for format the partition for use. For use as a network drive, you may as well give the drive a unique name that is memorable (Larry, Moe, Curly, etc) amd customize the icon for easy visual identification. Format the partition as MAC OS Extended Journalled, is what I did.
2. When your drive is USB, there is a built-in port on the back of the Airport Extreme. The physical plug in should be minor: ensure that you have a light on the drvie and a green light on the Airport Extreme.
3. With the physical connection done, configure the setting with Airport Utility. You will need to have the Airport Extreme device password to do this (you know: the WIFI password). Click on the Airport Extreme in the window to access the settings feature, then click EDIT to see and change the settings: On the DISK TAB check the box to enable file sharing. You will not need to change any passwords if you use Share with DEVICE PASSWORD on the menu from the DISK tab, settings page, and check to save the password into the user keychain. For more security and complexity(!), choose the Drive Password option or the USER Password access option (some people have un-trustworthy roommates or spouses).
NOW ACCESS the disk: in finder look for Airport Extreme under SHARED DEVICES in the left side column. Your drive should show up as a named folder with three little stick men on it. It may be designated as Kind=sharepoint (I never know why things are named what they are, because...well, usually sharepoint means something else). You may have to push a connect button and type in a password, based on what settings you chose, and what OS version you are running . Try not to push the EJECT icon. Try not to push the DISCONNECT button. Things stop working right, if you click on either of those things.
You can also get to files stored this way using an APP on your iPad or iPhone. This was my goal -> to make a 2 TB picture-music-pdf library. The APP I used was File Browser. There are others.