I purchased a cheap two drive USB enclosure in order to setup an external drive that had RAID-1 so I could backup photos and recordings.
First, I formatted both drives. Then I ran extended smart self-tests to ensure I had decent drives. With RAID-1 and two drives I can only tolerate 1 drive failure.
Next ensure mdadm is installed.
Determine which dev devices the disks show up as.
Next, create the raid device pointing at the correct dev directory.
The device will start a resync process which on my system takes a really long time (days). If you want to avoid this initial re-sync you can use '--assume-clean' to avoid this. I would recommend letting it resync.
And there ya go.