If you built raid into the kernel as modules
modprobe raid5

Create some nodes
mknod /dev/md0 b 9 0
mknod /dev/md1 b 9 1
mknod /dev/md2 b 9 2

RAID0
mdadm --create --level=0 --chunk=4 --raid-devices=2 /dev/md0 /dev/hde2 /dev/hdg2
mdadm --create --level=0 --chunk=4 --raid-devices=2 /dev/md1 /dev/hde3 /dev/hdg3

RAID1
mdadm --create --level=1 --chunk=32 --raid-devices=2 /dev/md0 /dev/sda3 /dev/sdb3

RAID5
mdadm --create --level=5 --chunk=32 --raid-devices=3 /dev/md0 /dev/sdc2 /dev/sdd2 /dev/sde2
mdadm --create --level=5 --chunk=64 --raid-devices=3 /dev/md1 /dev/sdc3 /dev/sdd3 /dev/sde3

Stop a raid build: mdadm -S /dev/md1

Watch the raid being built: watch -n 10 cat /proc/mdstat

mdadm --assemble /dev/md0 /dev/sda3 /dev/sdb3

mknod /dev/md0 b 9 0
fdisk /dev/sdb   g enter, n enter 1 enter enter enter, t raid, w enter
fdisk /dev/sdc   g enter, n enter 1 enter enter enter, t raid, w enter
fdisk /dev/sdd   g enter, n enter 1 enter enter enter, t raid, w enter
mdadm --create --level=5 --chunk=64 --raid-devices=4 /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

mdadm --manage /dev/md0 --add /dev/hdb1

mdadm --stop /dev/md0
mdadm -A /dev/md6 -m0 --update=super-minor /dev/sda5 /dev/sdb5

If you have more then 4 raids then you need to pass the others to the kernel as it will not find these automatically.

title Gentoo Linux 2.6.16-gentoo-r7.1
        root (hd0,0)
        kernel /kernel-2.6.16-gentoo-r7.1 root=/dev/md4 md=4,/dev/md0,/dev/md1 md=5,/dev/md2,/dev/md3