This run is very popular due to its proximity to the head of Little Cottonwood Canyon, as well as its relatively short length with multiple water features. Traffic is high on weekends and most mornings. If you want solitude, better to pick a day that isn't so perfect weather-wise.
Starting at the Granite Trailhead, you'll head up a relatively steep grade for about 0.3 miles (nothing too scary, but the difficulty may surprise if you're out of shape). After a few minutes of downhill, you'll head up to the Lower Bell Canyon Reservoir. There are trails around the reservoir, but stay to the left if you want to hit the waterfall.
You'll follow a dirt road around the left of the reservoir for about 0.2 miles before turning left, back onto the Bells Canyon Trail
(it's well-marked). Another half-mile and you'll be at a nice creek crossing with a wooden bridge and crystal clear mountain water. This is a great place to stop for a rest.
Continue on the trail into National Forest land for about a mile until you reach a trail junction at a large fir tree. It's not marked, but there is a well-worn trail that deviates to the left -- this is the side trail to the lower waterfall. Be careful here, as it's quite steep as you approach the falls, but it's definitely worth it. If you want to picnic, there's a great spot on some exposed rocks just below the falls.