1. Boot up with the Gentoo minimal cd.
2. Mount your gentoo partitions…
mount /dev/sda3 /mnt/gentoo
mount /dev/sda1 /mnt/gentoo/boot
mount -t proc none /mnt/gentoo/proc
mount -o bind /dev /mnt/gentoo/dev
mount --rbind /sys /mnt/gentoo/sys
mount --rbind /dev /mnt/gentoo/dev
3. chroot into the install gentoo environment…
cd /mnt/gentoo
chroot /mnt/gentoo /bin/bash
env-update
source /etc/profile