Decide on the location for the cable jack. Place the face of an outlet box onto the wall at that location and trace around the perimeter with a pencil. Drill a starter hole inside the perimeter you've traced and cut out the drywall to match the tracing with a keyhole saw.
Push your cable down behind the wall from above until you see the end of it in the opening. Pull out about 1 foot of cable. Insert the cable through the opening in the back of the outlet box. Place the cable box into the opening. Turn the mounting tabs on the outlet box with a screwdriver until they turn outwards and lock the box into place.
Screw the end of the cable onto the backside of a cable box faceplate. Place the faceplate onto the outlet box and attach it at the top and bottom with the outlet box's screws.