Most small mods (especially if they are called modlets) are just xml changes which is very safe and easy to install (no code from unknown sources and no change to the installed game files). Just create a folder "Mods" in the installation folder of your game and extract the mod (usually a zipfile) into that folder.
Don't forget that the Mods folder will not change when updating the game, so it is your responsibility to also remove it when it potentially becomes incompatible with a new version of the game.
If the Mod has a readme file, read it (obviously). If it says that you need to install anything to some other location outside the Mods folder then it is a more complicated mod and usually overwrites some files in the game. Then updating to a new version of the game usually means you must remove the mod, make sure everything works vanilla and then reinstall a new version of the mod. It also might mean you have to trust the mod creator.