NOTE: This article assumes that you have the necessary software installed in your GNU/Linux distribution.

1] Do a "su -", or do everything here as root.
2] You should have a Linux partition (hex code 83 if you use fdisk in GNU/Linux) for this purpose. Mine is /dev/sdb1
3] Do the following:

mkdir /encrypted

# Note: The following 2 commands will ask you for input.
cryptsetup -y -v luksFormat /dev/sdb1
cryptsetup luksOpen /dev/sdb1 encrypted

mkfs.ext4 /dev/mapper/encrypted
mount /dev/mapper/encrypted /encrypted

4] To unmount your encrypted filesystem:

umount /encrypted
cryptsetup luksClose encrypted

5] To remount your encrypted filesystem:

# Note: The following command will ask you for input.
cryptsetup luksOpen /dev/sdb1 encrypted

mount /dev/mapper/encrypted /encrypted