Moving to RAID was a pain.
What you have to do is the following from an existing install:
Install mdadm
Create your mdadm RAID 1 array on your spare hard drive.
Start it with the missing disk.
rsync the entire contents of your current / to the md partition.
Here's a good way of doing it:
rsync -Pha --exclude=/proc/* --exclude=/sys/* --exclude=/mnt/* /. /mnt/md2........