Unless something has seriously changed
you edit the /etc/fstab file. Must be edited as root (so run... sudo nano /etc/fstab)
You'll see the following format:
proc /proc proc defaults 0 0
/dev/mmcblk0p1 /boot vfat defaults 0 0
/dev/mmcblk0p3 none swap sw 0 0
dc4-arm-01:/home/grayg/stage/debpi_pubsd/root/mnt/opt/vc /opt/vc nfs defaults 0 0
or similar (this is from the old Debian image).
to explain a line.
Device block (ie /dev/sda1), mount location (ie /mnt/external) filesystem (ie auto - lets make life flexible), mount options (see below), dumping and checking priorities - leave as 0.
/dev/sda1 [\t] /mnt/external [\t] auto [\t] ro,auto,users [\t] 0 [\t] 0
[\t] = use a tab character not a space
ro = read only, you don't want this if you need to write to the drive (it's rare that you'd want it, but I thought I'd let you know it existed)*.
noauto = do not auto mount at boot.
users = allow users to mount and unmount the drive (if it wasn't plugged in at boot).
You don't normally have users without noauto, because most stuff that you mount at boot is expected to always be there. You would normally have noauto and users together as the user generally wants full read and write access to the physical device they keep in their pocket.
* Read up on the stability of NTFS writing under Linux - you may actually need the drive as read only after all.
Google and 'man fstab' can offer lots more.
you can then mount with