You should have Chinese visa in advance. Then when you in Kathmandu, you'd better to find a travel agent to do the next thing for you: China visa and passport copies to handle the Tibet Travel Permit and the permit will be sent to our local agent in Katmandu. You have to cancel the previous China Visa and then only can apply for a fresh China visa at the Chinese Embassy, Katmandu, Nepal based on Tibet Travel Permit. For any tourist entering Tibet from Katmandu, if he or she is holding China visa issued from their home country or any other place must be cancelled first. A bit complicated, isn't it? That's why I suggest you find a agency to do that for you. If so, you usually have to get to Katmandu 1-2 days before entering the high land. If you handle yourselves, it takes longer time. Visa Section at Chinese Embassy, Katmandu opens: 3 days a week, Mondays, Wednesdays & Friday.
The application submit time: From 09:00 AM to 12:00 AM
Good luck~