You need the kernel sources, obviously. And you have to edit the filename in the patch:
--- linux/arch/i386/kernel/setup.c-pre10 Sun Jun 16 21:34:48 2002
+++ linux/arch/i386/kernel/setup.c Sun Jun 16 21:50:53 2002
I removed the "-pre10" from the first line. Then
# cd /usr/src/linux-2.4/arch/i386/kernel
# patch -p4 < /path/to/patchfile/you/just/edited
Then follow the instructions to recompile a custom kernel (edit the /usr/src/linux-2.4/Makefile and add "Extraversion=<something>" so you don't overwrite your vendor kernel).
gbrauer mentioned the RedHat 2.4.18-10 kernel seems to have a (better?) patch for this, so you may not need this, but you might want to try it just to cover all bases.