I need to add a small patch to my kernel, so under Fedora 8 you can do this by:
yum install yum-utils
yumdownloader --source kernel
rpm -ivh kernel-2.6.23.1-42.fc8.src.rpm
rpmbuild -bp --target=$(uname -m) /usr/src/redhat/SPECS/kernel.spec
Another good resource is CustomKernel on the Fedora Project Wiki.
