In general, you just download the Java JRE or JDK from the java webpage:
and install it. Download - save somewhere - chmod u+x filename - ./filename (for the self extracting bin), or rpm -Uvh filename.rpm (for the rpm download).
Then, figure out where it installed (if you have to cd / ; find | grep java or whatever). That should contain the java, javaws, and all that other stuff. The plugin'll be in
Find out where your browser is (/usr/lib/firefox-2.0/ ??) and go in there, then into the plugins directory.
Remember, the simplest way to do a symlink and not screw it up is to always BE IN THE DIRECTORY WHERE YOU ARE CREATING THE SYMLINK, and to use this syntax:
ln -s [thing that already exists] [never put anything here]
Which will create the symlink in the current directory (this is why the java install instructions always say "DON'T FORGET THE LAST PERIOD"... it means "here, in this directory". It's not needed. Promise.
ln -s [java install directory]/jre/plugin/i386/ns7/libjavaplugin_oji.so