Not sure of the exact process but it needs to be done at a BIOS level, either within the main BIOS or in a secondary RAID BIOS accessed by a different keystroke after the main BIOS splash screen - your mobo manual will tell you how to access it.
Once you've got the array set up the system will treat it as a single HDD, installation of Windows is as usual with the exception that you'll need to have a floppy handy with the RAID drivers on it to allow the install to see the array - press F6 at the beginning of the install to allow you to load the drivers.
If you have any data on the drives at the moment it will be lost when the array is created, it isn't normally possible to migrate from a single disk to RAID0 while retaining data.