Step 1: Create Partitions on both(all) devices, these partitions must be identical in size and of partition type "FD" (Linux raid autodetect)

Step 2: Create arrays the following example is for creating raid1 on 2 drives (sda and sdb) with 4 partitions:

mdadm --create --verbose /dev/md/md1 --level=1 --raid-devices=2 /dev/sd[ab]1
mdadm --create --verbose /dev/md/md2 --level=1 --raid-devices=2 /dev/sd[ab]2
mdadm --create --verbose /dev/md/md3 --level=1 --raid-devices=2 /dev/sd[ab]3
mdadm --create --verbose /dev/md/md4 --level=1 --raid-devices=2 /dev/sd[ab]4

Step 3: List the newly created devices with cat /proc/mdstat (if they are syncing wait for them to complete the syncing process on all devices before proceeding)

Step 3: Format the new devices to ext4:

mkfs.ext4 /dev/md/md1
mkfs.ext4 /dev/md/md2
mkfs.ext4 /dev/md/md3
mkfs.ext4 /dev/md/md4

Step 4: Output updated mdadm.conf:

mdadm -Es >> /etc/mdadm.conf