First of all, make sure you have WinZip, WinRar, or some other type of tool that allows you to extract zip files. Almost every file is in a zip format, which is why you need the aforementioned program. A quick Google search should give you a download link. Just get the free trial of either program. You can buy the full version, but that's only if you want to get down and dirty in heavy-duty file making and extracting, which I'm pretty sure you don't want to do.
Open the zip file containing the mod with WinZip/WinRar and read the readme file that's included. Not all mods are extracted to the same folder, so you want to read the installation instructions that should be in the readme.
If by chance there is no readme file, or it's not particularly explanatory or clear, here are some suggestions on where to place particular files.
*If it's a custom player model or skin for MP, extract the file to your Jedi Outcast/Gamedata/base folder. After that, fire up JO MP and go to the character selection screen to activate it.
*If it's a custom SP level or a game modification for SP/MP that changes how the game is played, chances are you need to put it in Jedi Outcast/Gamedata under a separate folder. Once that is done, start JO and select it by selecting "Mods" under the directory "Setup." There are some simple SP levels that can be put right into the base folder, however. If that's the case, you have to load them by typing the map's name in the console menu. You can access the console menu from the game by holding "Shift" and pressing the "~" key (the one right by the "1" key).
I hope this helps. Have fun!