Lhasa is a trip you plan months ahead, and the order of the steps matters, because foreign visitors can't travel independently in the Tibet Autonomous Region of China. First comes a Chinese visa or visa-free entry for your passport. Second, a licensed travel agency, the only party that can apply for the Tibet Travel Permit. Third, the permit itself, which the agency files for you; allow several weeks, since sources disagree on how long it takes. Fourth, a guide who stays with you at the sights and on the road. The 240-hour transit rule doesn't cover Tibet. So this guide is written for an organized tour, and it separates what the tour fixes (permit, guide, driver, tickets, route) from what you decide (agency, group or private, hotel level, pace, how well you acclimatize). The first three tips below cover the paperwork in that order.
The reward is a place unlike any other in China. At about 3,650 meters Lhasa is the old capital of the Dalai Lamas, and the Potala Palace on its hill was their seat from 1649 until 1959; the Jokhang, begun around 640, is still the spiritual center of the city. Pilgrims walk the Barkhor circuit clockwise through the day, and monks debate in the courtyard of Sera in the afternoon.
Five days is a sensible length because the first one is for rest, and the altitude makes everything slower than it looks on a map. The plan keeps each day compact. Day one is arrival, sweet tea and a first walk around the Barkhor. Day two is the Potala and the Jokhang, day three the great monasteries of Drepung and Sera with the afternoon debate, day four a day trip to Yamdrok Lake over a pass of roughly 4,800 meters, and day five the Tibet Museum, Norbulingka and a last Barkhor stroll. Namtso, the other famous lake, is under More time, with the reasons. Most tours begin in Chengdu, the usual flight or train departure point, and the practical side of traveling in China, from payments to maps, is in the tips. Start on a Tuesday: the Potala and the Tibet Museum are closed on Mondays, and Sera's debates skip Sundays, so this order never touches a closing day.