Tibet tour from Kathmandu has different ways, highway or air line. In general, tourists would like to enter to Tibet by highway, which means you will go through Zham Port. If you are going to visit Lhasa, you will drive on the China-Nepal Highway (Sino-Nepalese Highway).
Tibet tour from Kathmandu could take the international air from Nepal to Lhasa. Kathmandu is the capital of Nepal, and it is the only one international air port of Nepal. There are four direct air lines of China to Nepal, Chengdu - Kathmandu (stop in Lhasa on the way) costs about 4.5 hours, Hong Kong - Kathmandu costs about 4.5 hours, Shanghai - Kathmandu costs about 5 hours, and Guangzhou to Kathmandu costs about 2.5 hours.
Nepal to Tibet overland is from Zham Port. Zham Port is opened at 9:30 am, and the officers will have a rest between 11:30 - 13:30. There is an international bus between Lhasa and Kathmandu.
There is about 8 kilometers long from the frontier inspection of Zham Port to the Friendship Bridge (connecting Tibet and Nepal). It takes you about 1.5 hours if you would like to walk, or you could rent a car to the Friendship Bridge.