You need special tools to do it. I just got the SnapOn MT1480 meter to do it myself. There is a port by the crank pulley that you place a probe into. You also place a pulse clamp on the #1 injection line. With the engine warm (cold timing advance off) rev it to 2000 rpm. The timing per ford should be 8.5 degrees before top dead center + or - 2 degrees.
Turn the IP to the passenger's side to advance the timing and to the driver's side to retard the timing. The IP's tend to become a bit retarded in timing as they wear.
I've had my truck timed before at the IDI rally's by Mel Agne. He does about a dozen or so trucks every rally for free. I'll have my meter at this years rally as well. If you can make it, you can have it timed for free.
Most shops don't have the equipment to time these IDI's and if they do, they don't know how to use them.
Heath