This is one of my favorite hikes in RMNP. It's a moderately easy hike if you're in decent shape, although you may feel the effects of the altitude (13000+).
From the trailhead, which cuts north off of the Bear Lake trail, you spend the first few miles in the trees. You'll gradually gain elevation, and to your right, you'll get occasional glimpse of Bear and Gem Lakes below.
Then you reach treeline. Watch for wildlife. It's not uncommon to run into mule deer, elk & marmot at this point in the hike. To the South, you'll have fantastic views of Dream & Emerald Lakes, within Tyndall Gorge.
From here, it looks like you must be getting close to the summit, but now you must hook around the North side of Flattop Mtn around Tyndall Gorge. It's worth it. There are dramatic views of North and if you look closely, you can see cars on Trail Ridge Rd.
Once you reach the summit (you'll see a sign... it's named "Flattop Mtn." for a reason.), spend some time looking around. To your South, you'll see Tyndall Glacier, and below it is Tyndall Gorge and Emerald Lake. Fantastic view.
You may choose to continue a 1/2 mile to the summit of Hallet Peak. I usually have a hard time resisting the urge to summit Hallet. It is one of the most striking features in this area of the park, and it is, after all, only another half a mile. Keep an eye on the weather, though. Storms crop up quickly in the Rockies.