Each mod should come with its own installation instructions, but most assume that you'll use a mod called Minecraft Forge which makes the whole thing a lot easier. Someone has created a mod loader called Forge

BACKUP YOUR GAME FILES

he most important files when it comes to mods is minecraft.jar. That is an archive folder that has all the important core minecraft files and folders inside it.Other important folders are .minecraft/saves and .minecraft/stats, which store data about your worlds and achievements. These three folders are the main ones you want to backup

Mac OSX ~/Library/Application Support/minecraft/

DOWNLOAD FORGE (follow these steps)

Go to the official Forge website (files.minecraftforge.net) Figure out which version you need (it needs to match the version of Minecraft that your mods are compatible with) Look for that version number under the Versions header If this is your first time installing Forge, pick the one that says Recommended Look for the link called Installer-Win (for PC) or Installer (for Mac) to download the installation file.

INSTALL FORGE

Once the file has downloaded (and you've scanned it with your antivirus protection software), double-click to open it and then select 'Install Client'.

LAUNCH MINECRAFT

Once Forge has successfully installed, run Minecraft and look in the profiles drop-down menu - you should see that you now have a new profile called Forge. Log in with that profile and click Play (wait for the green bar to fully load). You should now have a menu option called Mods. Exit Minecraft.

DOWNLOAD COMPATIBLE MODS

INSTALL THE MOD

LAUNCH MINECRAFT.