I bet this is already covered in some FAQ someplace. The recommended plan is to install a tiny linux system first. As well as being the easiest way to bootstrap (assuming you have disk space to spare). This gives you something else to use to mount hurd partitions and fix stuff if you ever get so the hurd won't boot.
Just do the most minimal installation of your favorite linux distribution, debian or whatever. You can install grub from the linux world too if you want. Then make your hurd partition(s) with mke2fs -o hurd, and unpack a base system tarball onto your hurd root partition.
