Setting up a (new) system
This section describes setting up a RAID system using a distro like gentoo or slackware, that expects you to do a lot of the work yourself.
Planning the layout
Linux sees most storage as block devices. It is also quite happy to layer block devices on top of each other. The devices we need to consider are disk drives, raid arrays, logical volumes, and partitions. Network Block Devices can also be used. It is very important to get clear which layers go where in the stack - linux couldn't care, but it can get very confusing for admins, and confused admins means computer disasters.
Partitioning your system
Every system needs a / partition. Most systems have one or more swap partitions. Many systems have a separate /home partition. You may want to have other partitions. And do you want to use LVM to manage your user partitions?
If you have a separate /home partition, do you want this on a separate array?
Do you want linux to manage the swap space directly, or do you want it on raid?
Setting up the boot disks
Grub needs the first 2MB at the start of an MBR disk, or a partition of its own on a GPT disk, to install itself. UEFI needs its own partition. That means you must partition your boot drives. If you put swap directly on the drives you run the risk of a drive failure taking out any processes that may have been swapped on to that drive. Putting swap on an array will protect against this.
Use fdisk or gdisk (or the partition manager of your choice) to prep your boot disk. If you are using GPT, create a 1MB partition, type EF00 for UEFI, or EF02 for grub. If you are not putting swap on RAID, add your swap partition (I always size my swap as twice the maximum memory the motherboard will take. On a modern hard disk, that's nothing). Put the remaining space into a partition to create your RAID from. If you are not using LVM you can create multiple partitions for multiple arrays if you wish.
Now create your arrays.
mdadm --create /dev/md/root --level=1 --raid-devices=2 /dev/sda3 /dev/sdb3
If you are using LVM, read the LVM howto and partition your array. I didn't, so
Now you can install your distro. Make sure you install grub on all the disks. You don't want to lose the first disk and suddenly discover your system won't reboot!
Setting up a separate home
It's a matter of preference whether to pass bare drives or a partition to the RAID if you're not booting off it. Some people think that's a bad idea, because installers assume a drive is blank if it doesn't have an MBR or GPT (as a gentoo guy, I don't run that sort of software). Others - including some notable raid names - prefer to use bare drives.
mdadm --create /dev/md/home --level=5 --raid-devices=3 /dev/sdc /dev/sdd /dev/sde