This trail starts out just up the road from the Hosmer's Grove Campground, where there are 5-6 parking spaces on the south side of the road. It is not marked, nor obvious, as you step off the paved road onto an open grassy field, that appears to have a fire road cut into it.
This is the trailhead; hop over the metal braided rope, and you'll quickly see the cut path of the trail. Most of the trail is through knee to shoulder high brush, on a fairly rocky singletrack. Adequate sun protection is a must, as there is no shade. It's composed of gentle switchbacks traversing the occasional volcanic rock gulley crossing, and does have a few short steep sections, including stair-like ledges of volcanic rock. Pack your lunch, as there is an open, fairly level spot at end of the trail where you can rest.
The trail has beautiful views out over northern Maui (if there are few clouds), and daunting views of the looming summit above. Halfway through this run, you may ask yourself "What's the payoff here?" Stick with it. As you come to the end of the Supply Trail, continue down the Halemau'u Trail
about 0.25 mi and amazing vistas of Haleakala's crater await. Prepare to be awestruck by its beauty and sheer vastness.