Create New RAID 1 Array:

First setup your partitions (make sure they are exactly the same size)
In my example I have sda3 and sdb3 which are 500GB in size.

mdadm --create /dev/md2 --level=1 --raid-devices=2 /dev/sda3 /dev/sdb3
mdadm: array /dev/md2 started.

Check Status Of The Array

*Note I already have other arrays md0 and md1.

You can see below that md2 is syncing at a speed of about 91MB per sec.

cat /proc/mdstat

Personalities : [raid1]
md2 : active raid1 sdb3[1] sda3[0]
      488287552 blocks [2/2] [UU]
      [>....................]  resync =  0.5% (2460992/488287552) finish=88.8min speed=91147K/sec
     
md1 : active raid1 sda2[0] sdb2[1]
      29302464 blocks [2/2] [UU]
     
md0 : active raid1 sdb1[1] sda1[0]
      58604992 blocks [2/2] [UU]
     
unused devices: <none>

Check the basic details of an array:

mdadm /dev/md0
/dev/md0: 55.89GiB raid1 2 devices, 0 spares. Use mdadm --detail for more detail.
/dev/md0: No md super block found, not an md component.

More Detail

mdadm --detail /dev/md0

/dev/md0:
        Version : 0.90
  Creation Time : Tue Aug 31 22:37:52 2010
     Raid Level : raid1
     Array Size : 58604992 (55.89 GiB 60.01 GB)
  Used Dev Size : 58604992 (55.89 GiB 60.01 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Wed Sep  1 18:39:53 2010
          State : active
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 7b4e9a42:823c7ee5:0d6ee1b8:30df68a8
         Events : 0.7

    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1

Update /etc/mdadm.conf File

mdadm --brief --detail /dev/md0

ARRAY /dev/md0 level=raid1 num-devices=2 metadata=0.90 UUID=7b4e9a42:823c7ee5:0d6ee1b8:30df68a8

To edit the /etc/mdadm.conf file directly just run the command like this make sure to use the ">>" or you will wipe out any existing array info.

mdadm --brief --detail /dev/md0 >> /etc/mdadm.conf