By having the overall shortest cumulative time for all of the combined stages. It isn’t even necessary to win any single stage; just being consistent often works. One guy may win today, then finish way back tomorrow, in which case his poor time cancels out his good, winning time. Inconsistency is the enemy of the Tour competitor.
I’m not at all an expert in this, but one’s team helps a lot in protecting the star by keeping him constantly in a draft position among them until the best time for breaking out comes. Sometimes they break out and go for a win or good placing in the stage, sometimes they just play it safe. Someone else with experience in racing can offer more detailed explanations of how one wins the Tour, but that’s my observation.
Thanks! That helps my understanding of the race quite a bit.