Get the kernel source from, ftp.kernel.org. Copy it to /usr/src, cd to /usr/src. Lets say the file name is linux-2.4.21.tar.gz, use command "tar zxvf linux-2.4.21.tar.gz" make a link " ln -s /usr/src/linux-2.4.21 /usr/src/linux " cd to /usr/src/linux, using vi edit the "Makefile" on line 4 you will see "EXTRAVERSION = " place the name of your kernel there eg " EXTRAVERSION = linux-2.4.21Custom" save the file and exit. Now run the command "make config" you will be presented with a text menu, select the hardware that suits your machine and the kernel parameters you require. Next run " make dep; clean; bzImage; modules; modules_install " this could take a while depending on your machines spec ( I once had to leave this part over night on a 486 with 32MB RAM! ) Next "make install" when this stage is complete check your boot loaders configuration file, if it's grub.conf you'll find it's been edited automatically, if it's lilo.conf you will need to edit it yourself and on completion use the command " lilo -v " reboot and load you new kernel by selecting it's name from menu presented by the boot loader.
I suggest that you go to www.tldp.org
and read the kernel-HOWTO and repeat the steps until you can do it blindfolded!!