The Kalaupapa trailhead is on the east side of Hwy 470, just north of the mule stables, and is marked by the Palaʻau park sign and parked Kalaupapa employee cars. The 3-mile trail has 26 switchbacks, 1400 steps and drops 1664ft in elevation from start to finish.
It's best to begin hiking by 8am, before the mules start to go down, to avoid walking in fresh dung, though you have no choice on the return trip. Allow an hour and a half to descend comfortably. It can be quite an adventure after a lot of rain, though the rocks keep it from getting impossibly muddy. Many find walking sticks a huge help.