First, be sure to close out Minecraft. This is essential. If you reach any errors, ensure that minecraft is fully shut down, and nothing is opened in any other programs such as modding tools like INVedit.
1. Locating your Minecraft.jar
Go to your %appdata% folder by typing "%appdata%" into Windows explorer or into Run(Win+R)
This should take you here: Make a backup of the .minecraft folder! Copy it to your desktop, put it in a zip or rar, but be sure to back up!
Go into the .minecraft folder... you should see this:
Now enter the bin folder:
2. Opening the .jar
Simply right-click the minecraft.jar, go to Open with, then click on 7-Zip File Manager
You should see something similar to this folder:
3. Installing the mod
Download whatever mod you wish to install and locate the .zip .7z .rar or whatever it came in. Sometimes this might be your download folder! In this example, it's a .zip. Open it, and place it next to the minecraft.jar 7zip window.
Now simply drag all of the class files, you can exclude any readmes(though you should read them!) into the minecraft.jar.
And click Yes
4. Deleting the META-INF
Still within the 7zip window, enter the META-INF folder and delete any files that start with MOJANG, such as MOJANG_C.DSA and MOJANG_C.SF. Do not delete the MANIFEST.MF.
Now close all 7zip folders and test the mod out in Minecraft: if it does not work revert to the backup you hopefully made previously. Most mods will work if they are the only mod you install, and this isn't after a major update, but if you double or triple-up on mods there may be conflicting .class files, breaking the mods or your game.
I'm using 7zip and have winrar, and have minecraft closed, but when i drag the .class files into the minecraft.jar 7zip folder, it replaces my cursor with a little circle with a line through it icon. what do?