often called "interrupter brakes" , here is a video showing how to install them. I have used them for six years and really like them on my drop bar bike. I angle mine down just like on my mtn bike as the levers fall naturally to my fingers when I ride on the top bar--the advantage of angling them down is also that my handlebar bag clears the levers easily and I can still use them with my Ortlieb bag on with no problems whatsoever.
Ask a Mechanic: Installing secondary ?cross brake levers