How to frame a rough opening on an existing header?
It depends on the window size to some extent, but typically I'd not add another header. In fact, some carpenters put all headers against the top plate, with pin studs down to the RO.

Just drop your studs in and build down (if necessary) by whatever means is convenient. You always want at least two members along a door or window for rigidity, curtain backing, etc. Tie it all together well with sheathing and be happy.