Go to
www.kernel.org and download the latest stable kernel sources(bz2). If your patch is for an older version of the kernel, download that version. bunzip2, untar (tar jxvf, in one step) into /usr/src/
Create a symlink 'linux' to point to 'linux-x.xx.xx' . Apply your patch.
My /usr/src looks like this.
lrwxrwxrwx 1 root root 12 Dec 10 22:19 linux -> linux-2.4.20/
drwxr-xr-x 15 root root 4.0K Apr 23 13:19 linux-2.4.20/