I just recently swapped the old threaded fork out on my GTB.
1) Purchase the correctly sized headset
2) Disassemble existing headset
3) Knock the old cups out (friendly LBS who lends you the tool a plus here)
4) Install new cups and crown race (I used the DIY headset press)
5) Assemble with uncut fork to determine correct length
6) Double check your measurements
7) Check the measurements once more
8) Cut the fork to length
9) Assemble and ride
Whole operation took me about 3 hours, including a trip to the hardware store for the making of a DIY crown race press.
This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.