If you downloaded a new kernel from kernel.org, now it's time to unpack it. To do so, cd into /usr/src. If there is an existing "linux" directory there, move it to "linux.old" (mv linux linux.old, as root.)

Now, it's time to extract the new kernel. While still in /usr/src, type tar xzvf /path/to/my/kernel-x.y.z.tar.gz or cat /path/to/my/kernel-x.y.z.tar.bz2 | bzip2 -d | tar xvf -, depending on whether your sources are compressed with gzip or bzip2. After typing this, your new kernel sources will be extracted into a new "linux" directory. Beware -- the full kernel sources typically occupy more than 50 MB on disk!