It is very complex if you want to enter Lhasa from Nepal, the difficulty is not the plane tickets into/out of China, the most difficult thing is the Tibet Travel Permit, you have to get a Tibet Travel Permit when you apply for China visa in India, you can contact a China Travel Company(https://www.travelchinaguide.com/) to arrange the permit for you. In fact, the China Travel Company will asks you to apply for a China Visa in India first, then send the photocopy of your passport and China visa to the travel company, the company will get the Tibet Entry Permit for you, then send the permit to a travel agency in Nepal which has cooperation with it, when you arrive in Katmandu, Nepal, the travel company in Nepal will pick up you and tour guide in Katmandu will collect your passport and get your signature on filed up form to apply for a new China visa for you.

As a matter of fact, you have to apply for China visa twice if you enter Lhasa from Nepal.

In this case, you'd better enter other places of China first, then have a travel company to arrange your tour to Tibet. It will be much simpler in this way no need to apply for the China Visa twice. Please bear in mind that don't mention your tour to Tibet when applying for the China visa in India.