create a title with black "boxes" top, left, right and bottom. Put this at the top of the video stack
In PPro 2.0 you can import a letterbox matte. Open the titler then go to "title" > "templates". Then choose "Title Designer Presets" and go down to "Mattes". Choose "matte letterbox" for that perfect letterbox look every time. Keep in mind that this is only good if you are working on a project in 4:3 and want it to be letterboxed.