Converting an existing system
Much of the work converting a system is very similar to setting it up. The big question is are you upgrading all the drives, or are you adding a similar drive to the existing one, and you want to make it a mirror. he approach is similar in both cases. You have read Setting up a (new) system?
Plan your new drive layout, install your new drives, and create the filesystems, mirrors, and lvms the same way as setting up a new system. The only real difference is that if you are only adding one new drive, you haven't got two drives to create your raid, so the command to create the array(s) is slightly different:
mdadm --create /dev/md/root --level=1 --raid-devices=2 /dev/sda3 missing
Note the use of "missing" in the place of a device to build the array on. This tells mdadm to reserve a place, but that you don't yet have a drive to put there, and it will create the array in degraded mode. obviously, once you've move your system across to the array, you will add the existing drive to get a fully functional array.
Checking your kernel
The first thing to check is, does your kernel support raid? Most modern distro kernels will support it out-of-the-box. The easy way to check is
anthony@ashdown ~ $ cat /proc/mdstat Personalities : [linear] [raid0] [raid1] md125 : active raid1 sda3 sdb3 100597632 blocks super 1.2 [2/2] [UU] md126 : active raid1 sdb4 sda4 100597632 blocks super 1.2 [2/2] [UU] md127 : active raid1 sdb5 sda5 2694204672 blocks super 1.2 [2/2] [UU] unused devices: <none> anthony@ashdown ~ $
Look at the "personalities" line - this kernel supports linear, raid0 and raid1. A distro kernel should also support raid4/5/6 - this is gentoo and 4/5/6 wasn't compiled in. If your kernel doesn't support raid, it's easy to download the distro kernel source and compile it in, but you will have to watch out that an update does not remove it again.
Secondly, check your boot configuration. Most distros no longer boot straight into linux, but boot into an initramfs. This is mandatory if you want to boot off a modern superblock. The last few lines of your menuentry in grub.cfg should look something like this:
echo 'Loading Linux 4.4.6-gentoo ...' linux /boot/vmlinuz-4.4.6-gentoo root=UUID=ab538350-d249-413b-86ef-4bd5280600b8 ro domdadm echo 'Loading initial ramdisk ...' initrd /boot/initramfs-genkernel-x86_64-4.4.6-gentoo
Note the initrd line - if this is missing and your last line is similar to the "linux" line, then your system will not boot off raid. You will need to configure your distro to use an initramfs.