Even if you don't absolutely have to do this in order to get a kernel that supports your hardware, it's worth doing it in order to get a kernel that doesn't contain masses of drivers for stuff your machine doesn't have -- and even more so, in order to get a good understanding of how your kernel configuration options affect the behaviour of your system! As linux grows, the ability of the distribution vendors to configure a one-size-fits-all kernel has become very limited. For details on how it's done, see the Kernel HOWTO; it looks complicated at first, but it's not rocket science; just take it a step at a time.
Install the rebuilt kernel on a floppy at first; once that boots ok, install on the hard disk, run lilo if you're using it, and reboot.