When partitioning the drives with fdisk or gpartd, etc, instead of setting them to FAT, or EXT2, or whatever, set them to Linux Raid, which I believe is code "fd".
After you have the raid setup, you mkfs the type file system you want on it the /dev/md0.
You may find it helpful to
to watch the rebuild happen after you initially create the drive.
You can, of course, start using the drives right away, but if you are loading an bootable os on them, confusing things can happen if you don't give it a good head start on building.