These are absolutely the worst phones to learn on. You either need to install SCCP/Skinny on Asterisk (which FreePBX has no direct support for) or you need to flash them to SIP and lose a bunch of standard features because these phones were 100% designed for the Cisco UCM first and general SIP as an after thought. Now you can get a special patch for Asterisk that makes those features work with Asterisk but it’s a 3rd party patch and you have to recompile Asterisk for it. The maintainer of the patch does a good job of keeping it to as current as possible with Asterisk LTS releases (13 and 16).
The next bump in the road with these phones, they do not have a way to configure them directly. They do no have a web GUI management interface. So you just can’t log into the phone and throw in the settings you would like. You need to have a provisioning server (the PBX will suffice) and you need to create the config files for the phone to use and then tell the phone (via the phone interface) where to pull those config files from so you can get the phone programmed.
The Cisco 79XX is a dead line of phones and part of the reason they are so cheap on Ebay and the like. However, that being said someone like @cynjut, who loves these phones, can probably give you the better step by step to making them work one way or the other (SCCP or SIP).
Personally, if you wanted a phone to learn SIP on you were better off getting a low end Polycom, Yealink or something along those lines. A phone that is a SIP phone first and follows the SIP standards vs a phone designed for a specific PBX that doesn’t follow SIP standards (it’s Cisco, they have their own version of things as usual).