WHS does not support RAID from within.
Since WHS has a Redundancy system RAID is Not really necessary.
However, if RAID makes you "Feel Good", you can put a standalone RAID card and configure it work through Hardware.
You have to configure the RAID and format on the computer before you install WHS.
In the process of the Installation you would get one time a BSOD. At that point you will have to restart and provide the original RAID drives to the system (WHS is a subset of Windows 2003 SBS so when Win 2003 kicks in during the installation it needs the RAID drivers).
Once it is Ready install WHS (it would recognize it as a single drive) and copy the data (if needed) to the Data partition of WHS.