volume scalling for a twin reverb

[Bruce Night]

hi to everybody, i'm kind of new here (actualy the first thread!) and i'd like some help.
i have a twin reverb 100w and i want to add a swich to scale it at 50w and 25w something like the thread for orange tt.

could anybody, please, give me directions about how to do it? some schematics etc. thank you very much...