In Chapter 9 as you work towards the infamous dress scene, you end up performing a Dance routine with another character.
Make a save just after entering the Honeybee with Aerith, before you approach 3 Bees, aftet the tutorial and a cut scene the dance starts, there will be a total of 28 button presses.
My score a modest 20 Great, 4 Good and 4 Bad.
After this another quick scene and you're unveiled in full dress outfit, the Trophy popped at the final reveal after I got a notification that I had acquired some earrings.
If it doesn't unlock (my previous attempt with just 19 Great it didn't) then reload and try again.