Make sure your bomb-bay doors are open, then calibrate the bombsight according to the instructions at the top left. After all text is green, you can steer with left and right deflection of your joystick.
While holding the Numpad 8 key down, use the arrows and Page Up/Page Down keys to get an unobstructed view out of the nose of the aircraft, then press F-10 to save it. You can now toggle that view with the 8 key while in the bombsight view and steer toward your target, zooming in (Z, then [ and ] keys) if necessary.