Download and extract a 2.6.18.8 kernel tree.

wget http://mirror.kernel.org/pub/linux/kernel/v2.6/linux-2.6.18.8.tar.bz2

tar xjf linux-2.6.18.8.tar.bz2

Clone a clean xen linux-2.6.18-xen.hg tree.

hg clone http://xenbits.xensource.com/linux-2.6.18-xen.hg

Diff to create a patch.

diff -urN -x .hg linux-2.6.18.8 linux-2.6.18-xen.hg >linux-2.6.18-xen.patch


see main howto and subsitute above patch from the make linux-2.6-xen.patch

There are a few more merge conflicts to resolv.