First you will want the Supreme Commander 2 mod manager which you can get from moddb: http://www.moddb.com/mods/dlc-eternal-conflict/downloads/cerus-vis-supreme-commander-2-mod-manager

Second thing you need is the mod you want to install. Choose whatever one you want, and download it. That's all you need really! :D

The mod you download should be in a .rar file. Open it up and drag the .scd file to your desktop. This is what will be the mod you put into the mod manager.

The mod manager should also be in a .rar format. Open it up and open the .exe. At the bottom, it should say 'add'. Click it and it'll bring up a browsing window. Click 'desktop' and find the .scd we put there. Click it and choose 'open'. The mod should then show up in the manager. To play Supreme Commander 2 with the mod, simply click/check the box next to the mod and click 'launch'. The game should then open with the mod enabled. The way to double check is to look at the bottom of the main menu and it should say '*The mod you chose* is enabled' or something along those lines.

Congratulations! If you followed everything correctly and it all worked out fine, you should have your new mod installed and ready to play. If you have any questions or things I did wrong to point out, please pm me or tell me in the comments section. This is my first guide so I would like some recommendations and/or suggestions. :) Again, thank you for reading and have fun with your new mod(s)! :D