I start by picking the groupset I want then build the rest around it, finding a frame of the same period etc. Sometimes I've been lucky and found a whole 'survivor' bike other times I've built up from bare frame.
I currently have an early 90s 8 speed 6400 STI groupset that needs a frame. Previous builds have been Campy Super Record, Campy C Record with Deltas, Shimano 6400 7 speed 1st release, Shimano 7403 8 speed STI.
I'm thinking 7200 Dura Ace next...