1) install either the raid-toolkit or mdam software packge
2) recompile the kernel to include the raid levels you want.
3) create the /etc/raidtab file ( RAIDTAB examples
4) I suppose you should have installed the hardware first. My bad.
5) create the array, which will depend on if you use mdadm or the raidtools.
Are you installing the system onto the raid5, or creating a raid5 array to store files? If you want to mount /
onto the raid5 array, I'm guessing you'll have to re-install your system, or image it to the array.