Power on or restart your Mac.
At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
Type sh /etc/rc and press Enter/Return.
-- Wait for this to complete, ~ 5 min (no progress bar or indication that it is completed)
Type passwd username and replace username with the short name of the user account for which youd like to reset the password.
(If you don't remember the shortname, type ls /Users)
Type your desired password and press Enter. It wont show itself on the screen, so be careful what you type. You will most likely have to enter it again to confirm.
Type reboot and press Enter.