d/l = download, damn the internet has played hob with my typing.
Grab the kernel source from www.kernel.org,
or a mirror that is close by, the whole kernel source, compressed, is about 30Mb.
Make certain you have a compiler installed, RedHat doesn't like to give you one unless you chose "developer" or "everything". Weenies. To find out:
That should tell you the version of the Gnu C Compiler you have, or return "command not found". Whatever GUI package manager you've got should be able to handle the 2-3 dependency RPMs it has.
As for a how-to, the Linux documentation project. Its pretty much the end all be all of kernel how-tos: