Because you're using such an old version of DNN, you run the risk of uploading a skin that won't work. However, if you're pretty sure the skin is suitable then carry on. It certainly won't damage anything.
Another issue is that skin developers don't always follow good form when packaging their skins. Check for zips inside of zips and look for a zip that hopefully contains a couple of folders called Skins and Containers. That is the zip you will upload. When I first started in DNN on 2.1.2 I only found out through trial and error that everything from modules to skins and containers is just a huge mess... just bear with it and try and look for reputable skins and modules. I recall finding skins and modules spread all over the server because of incorrectly made zips.
There is a Skins menu item on both the Admin and the Host menus. You can upload most html-based skins to either. A Skin and a Skin plus Containers zip are both uploaded using the Upload Skins menu that pops up when you hover on the little triangle on the skins page (all highly intuitive heheh). whereas a zip with just containers goes into the Upload Container section off the same menu. I'm pretty sure I recall this right.
If the folders in the zip are all correct and have the proper files in them, then after upload, any html files present will be processed by DNN into equivalent ascx files. Those will then appear on your skins listing. Clicking the "Parse Package" link on the skins page actually re-does this processing again for you on the server. You can use that if you have made any changes to the html files whilst they are on there. Watch the parse process to see that it doesn't produce any error messages in red.
The point is: DNN skins itself using the ascx files. These are created from the html files (if present) upon uploading or upon manually parsing the skin if already uploaded.
Have a further fiddle and come back here when you get stuck again. It's a nightmare to figure out.
Rob