Well, you need to start off in the following directory on your hard drive:
C:\Program Files\LucasArts\Star Wars Battlefront\GameData\Data\_LVL_PC>
If you do not know how to do this, go to your 'my computer' icon in the start menu, click on the C: drive, then each of those subdirectories in turn:
Star Wars Battlefront
Within that _LVL_PC and its subdirectories you will find all of the files that end in .lvl
Those files are what are being modified. Now, if you are downloading a Hoth mod you need to click on the HOT file folder, find the HOT1.LVL file, right click it, click on copy and then paste that file into some folder you plan on keeping your good, unaltered files.
When the time comes that you are done with the mod and you wish to play the original game again (or if you wish to play online, as you cannot have most modifications if you are playing online) you can then copy that original .lvl file you've saved and paste it back into its original folder. It will ask you if you want to replace the one that is in there and you say YES.
Conversely, you can rename the one that is in there to 'HOT1 Modded.lvl' or something similar if you wish to so that you do not have to re-download it again.
Lastly, if you do somehow mess up your game and cannot get it fixed, fret not.. you can always uninstall and then reinstall a clean copy of the game onto your hard drive.