setting up luks on a new partition

gistfile1.txt

loadkeys uk
lsblk
gdisk /dev/sda
  outout should eventually look like:
  Command (? for help): p
  Disk /dev/sda: 
  Logical sector size: 512 bytes
  Disk identifier (GUID): 
  Partition table holds up to 128 entries
  First usable sector is 34, last usable sector is 
  Partitions will be aligned on 2048-sector boundaries
  Total free space is 2014 sectors (1007.0 KiB)

  Number  Start (sector)    End (sector)  Size       Code  Name
     1            2048            8191   3.0 MiB     EF02  BIOS boot partition
     2            8192         1032191   500.0 MiB   8300  boot
     3         1032192                               8300  root
cryptsetup -v --cipher aes-xts-plain64 --key-size 512 --hash sha512 --iter-time 5000 --use-random luksFormat /dev/sda3
  enter a passphrase twice here
cryptsetup open /dev/sda3 ARCHROOT
mkfs.ext4 /dev/sda2
mkfs.btrfs /dev/mapper/ARCHROOT
mount /dev/mapper/ARCHROOT /mnt
cd /mnt