partition your /dev/sdb similar to /dev/sda. I personally would use fdisk
Another user on here, i_grok, helped me and also mentioned that you could use the sfdisk command, but I haven't tried it. It's suppose to replicate the partitions to the other drive. For example,
sfdisk -d /dev/sda1 | sfdisk /dev/sdb1
After, create the initial /dev/md0, /dev/md1, etc...
mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda1
mdadm --create --verbose /dev/md1 --level=1 --raid-devices=2 /dev/sda2
Now sync up /dev/sdb with /dev/sda
mdadm /dev/md0 --add /dev/sdb1
mdadm /dev/md1 --add /dev/sdb2
check the progress of the syncing
You can always find more information
I could be wrong about all this, though. I've only done the software RAID once or twice and am recalling this from memory.
You can also google up this question as there are tons of articles online that will explain this