what is the easiest way to travel Mainland China-Tibet-Kathmandu, return back the same route

Well, first of all, you have to obtain the Travel Permit in hand before you entring.
However, it's so difficult for you to apply it. Besides, foreigners are not allowed to enter alone. Thus you need to join in a group tour of local travel agencies. The staff will help you get the permit and plan the sightseeing itinerary.