I'm assuming you know how to physically install it, find a bay--connect the power--connect the IDE cable. You need to check the jumper pin settings on your current hard drive. It will most likely be set to MASTER. You'll need to check for a MASTER WITH SLAVE setting.
On the 2nd drive set it's jumper pins to SLAVE.
There should be labels on the drives that show the various settings.
Power up and the system will assign a drive letter. If you have other drives, (ie: CD's, DVD's, etc), you will want to check to see if their drive letters change. Some of the applications that use those drives might get a bit confused.
Good luck