Start at Keoneheʻeheʻe Trailhead in the Haleakalā Visitor Center parking lot and cross the crater toward Halemauʻu. The complete route is a full-day hike, and there’s no park shuttle between the two trailheads. For a one-way crossing, the park suggests staging your vehicle at Halemauʻu and arranging a ride up to the start. Bad weather can make that ride harder to find, so sort out the car plan before setting off.
If you’re considering turning around, remember you still have a substantial climb back to the rim. The Sliding Sands trail drops almost 2,500 feet to the crater floor; that downhill start can make the effort feel easier than it is.