Creating a software RAID, using e.g. the Debian installer is a breeze. Even with >2TB disks, which require using GPT as partition table (MS-DOS partition table is deprecated).
However, I decided to create the soft-RAID after the installation, which is a bit trickier and different than how it used to be using fdisk for <2TB disks.
Thanks to others, I've had a list of documentation to help me out:
>2TB Linux SoftRAID from commandline
>2TB Linux SoftRAID from commandline
Jumping out of an airplane is not a basic instinct. Neither is breathing underwater. But put the two together and you're traveling through space!