Ok,
Step by not-so-detailed step.
-- Put a computer (that is not the computer in need of the testing) into target disk mode. (hold 't' just after the startup tone)
-- Connect the two computers (one in target disk mode and the other as the one needed to be 'fixed') with a 6 pin firewire cable.
-- the computer in target disk mode will look like a firewire drive to the other.
-- reboot the other (not the target disk mode computer) and hold down the option key, this will pull up a menu to boot from.
-- Boot off the firewire disk shown
-- now you will see the other computer's desktop and your local drive mounted. Unmount your local disk.
-- run tech tools and anything else you need.
Hope that helps.