How do you install a mod for Minecraft 1.3.1?
You download the mod and open it with a archiving program,
Win-rar is what I use but there is also 7-zip, then you have to
find your .minecraft folder. You can find it by searching %appdata%
in your start menu. Open the .minecraft folder then open the bin
folder. Inside the bin folder, there is the .jar. Open it with a
archiving program. Look at the mod. If there is a Readme file, open
it and follow what it says to do. Also, there is usually the
instructions on the page that you found the mod. If there isn't a
Readme folder, then open the mod with the archiving program and
copy all of the .classes into the .jar or your .minecraft
folder.
Caution: If you get a black screen when opening minecraft after
installing the mod, then it is because you didn't delete the
META-INF folder in your minecraft jar.
Hope this was helpful. Ask me if there is any other
problems.
-Aiya