Just changed the rear brake drums on the 60s. Unfortunately they're squealing a bit.

Where on the rear drum brake system should you put copper grease to quiet them down a bit?

no where at all. it goes on the tips of the shoes where they touck the cylinder piston and the other end on the other end and on the bit of the back plate that the shoes rub on

Like Dave says only on the moving parts esp the pivot bar which only moves a fraction.
The shoes will bed in and stop squealing in a while.
Pete