Take trifluoroacetic acid, react it with SOCl2 to form trifluoroacetyl chloride, and then react that with trifluoroacetic acid to form trifluoroacetic acid anhydride and HCl gas.
Yuo can use the general method from my page which uses the same method to form ordinary acetic anhydride.
http://rhodium.lycaeum.org (http://rhodium.lycaeum.org)