Dear Amitava Dasgupta,
Greetings from TopChinaTravel! Thanks for contacting with us.
As Thailand and China are not adjacent to each other, you have to reach Lao or Burman first then from where take driving to the border and enter China from, for example, Xishuangbanna. From Xishuangbanna you can take shuttle bus to Kunming, which will take you about 10 hours, or you can also choose flight.
If you wanna go to Nepal via Tibet from Kunming, you’d better flight to Chengdu first for a stopover then flight to Lhasa (Tibet).
Zhangmu is the port of China while Kodari locates in Nepal. Arrival Zhangmu, you just go through the formalities of China Customs and apply for your Nepali visa, then you need to cross the Friendship Bridge to Nepali side.
Any further inquiry or question, please feel free to contact with us.
Kind regards,
Zoe