Quick newbie guide to rebuild kernel in FC3
 

1. Install source rpms
From terminal type

up2date --get-source kernel

this command will download and install kernel sources
if doesn't work (in my case it didn't) get src.rpm manually from
ftp://mirror.linux.duke.edu/pub/fedo...81_FC3.src.rpm
NOTE!! Use this kernel source only if your kernel package is updated via up2date
then

rpm -ivh kernel-2.6.9-1.681_FC3.src.rpm

2. Navigate to SPECS directory

cd /usr/src/redhat/SPECS

type
rpmbuild -bp --target=i386 kernel2.6.spec
(you can change target to i686 if you like :) )

3. Navigate to BUILD directory

cd /usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9

type
make xconfig
this will load default configuration into kernel editor and take your time to tweak its settings.
Be careful here and ask any questions if you're in doubt.
save/exit

4. Building kernel

type
make
takes around 15 min. on Athlon 3200. When it's done

make modules_install
in the end

make install
reboot and select new kernel from grub (usually kernel-2.6.9-prep)

Good luck!