The Overmountain Victory Trail (OVT) trailhead is near the Bandit's Roost (BR) parking area. You'll head East (roughly) on the OVT and ride a couple of miles on it to the Berry Mountain Park, crossing a wooden bridge over the actual Horton's Holler trail.
Turn around at the Berry Mountain Park road and head back West on the Horton's Holler trail. This portion is also called the Lower Berry Mountain Loop and runs along the reservoir shore in places. It's very bermy and twisty, steep in places. Loads of fun.
You'll go under that wooden bridge now and follow Horton's Holler trail to the upper loop, aka Upper Berry MOuntain Loop. More twisty bermy fun, but away from the lake.
This will come back onto the OVT, where you'll turn to return to BR. Each of the Horton's Holler loops is about 2 & 1/4 miles.