Upon arriving at Muzdalifah after sunset from Arafat, the first priority is combining Maghrib (3 rak'ahs) and Isha (shortened to 2 rak'ahs) prayers. The Prophet (peace be upon him) combined these two prayers at Muzdalifah with one adhan and two iqamas. This should be done soon after arrival — do not delay the prayers unnecessarily to settle in.
The requirement for staying at Muzdalifah varies by school. The Shafi'i and Hanbali schools consider staying at Muzdalifah even for a moment after midnight as fulfilling the wajib (obligatory act). The Maliki school requires staying most of the night. The Hanafi school considers the stay at Muzdalifah as wajib, and departing before Fajr without excuse requires fidyah. The strongest practice is to stay from arrival until after Fajr prayer, then depart for Mina before sunrise — this is the Sunnah of the Prophet.
During the night, collect 49 pebbles (or 70 if staying in Mina for 3 days of Tashreeq) for the stoning ritual at Jamarat. Pebbles should be slightly larger than a chickpea. Sleep under the open sky — there are no tent structures at Muzdalifah. Make dua and dhikr, especially at the Mash'ar al-Haram area. The night at Muzdalifah is a profound spiritual experience of simplicity and equality, as all pilgrims — rich and poor — sleep on the bare ground together.