How to get the Invitation letter from Chinese Tourist Bureau

I wish to travel to China in 2015 November with my family (4 members ).We like to go to Tibet.From Xining to Tibet by train and from there we want to have a tour guide for 4 nights then return to Xining by train.
Please let me know the exact procedure to follow to obtain Invitation letter from China Tourist Bureau and also Tibet Entry Permit. Your help is highly appreciated. Thank you.