Note that the trail is not maintained past the Basin (approximately 2 miles in). Only experienced hikers should attempt the full hike to the Flatiron as the trail is steep and difficult past this point.
The first mile is misleading and not enjoyable. As you reach the canyon opening you can get a good sense of what is waiting for you. About a quarter mile into the canyon you'll reach Siphon's Draw.
The climb up the draw is slippery but taking your time makes it easy. Remember on your way down to zigzag to make the descent easier here. There's a waterfall to your left here if there has been recent rain. You are just shy of the halfway point and this is a turn around point for a lot of people, but why stop here?
The trail now becomes more of a scramble. Stop often to turn around and enjoy the view and to get a picture of your path down. Near the top, you'll come to a section that requires upper body strength to pull yourself up or having someone push you. This is the last real obstacle before the trail turns right and out to the tip of Flatiron. Take a break and enjoy the view. You've earned it!