If you choose to travel from Nepal to Tibet, you can choose to take airplanes or an overland tour. For entering Kathmandu, Nepal, you need to apply for your Nepal Visa on arrival. You can get your Nepalese visa on arrival when you reach Kathmandu's Tribhuvan Airport, road borders at Nepalganj, Birganj/Raxaul Bazaar, Sunauli, Kakarbhitta, Mahendranagar, Dhangadhi, and the China/Tibetan border - the Rasuwagadhi checkpoint.
Actually, Nepal is the closest overland gateway to Tibet. The linear distance between Kathmandu to Lhasa is barely 600 kilometers or 373 miles. The overland distance is 1,000 kilometers or 621 miles. If you choose this option, you need to plan at least 3 working days in Kathmandu to apply for your China Group Visa, which is the requirement document if you enter Tibet from Nepal. Even if you have already obtained your Chinese Visa, it would be invalid if you go this route.