1) First look into or explore the files in the patch, don't unzip it, and write down the file names and extensions that will be replacing the original game files.
2) Find the yellow folders in the main game folder containing the files that will be replaced by the modded files.
3) Back up your old files by creating a new yellow sub-folder in each yellow folder within the main game folder that has files that will be replaced by the new modded files. I usually call this new yellow subfolder Original Files or Original Flag Files, Original Data Files, whatever is appropriate.
4) Create a new yellow folder on your desk top, and this could be done in 1), and unzip your new modded files into this new yellow folder, called New Mod FoF Files or whatever.
5) Then maually copy each new mod file into the appropriate main game yellow subfolder to replace the original game files that you previously into new yellow Original File subfolders.
For mods without a lot of new files that is how I do it. For vastly more complex mods with many new mod files, I make my Original File sub-Folders and put the original files into them that are going to be replaced by the new mod files, then I just unzip the main mod file into the game, or however the mod read Me file in the mod tells me to do.
If you goof-up, as we all have doing this, just reinstall the game and try again with adding the mod files.
Some people make copies of the whole game for each mod, calling each complete game copy something different such as FoF, FoF1, FoF2, FoFBigMod, whatever.
Just experiemnt and you will pick it up fairly quick.
That's about the easiest manner that I can explain this.