Measure the height of the pocket side jamb on both sides with a tape measure. Cut one of the 1-by-4 boards to the measurement with the circular saw. Measure from the edge of the wall to the edge of the pocket on each side of the pocket door frame.
Transfer the measurement to the cut board and rip the board to the measurement with the circular saw using a rip blade so that you have two trim boards. Since you rip the length of the board you will be cutting with the grain. Using a rip blade prevents the board from splitting and also helps ensure a clean cut line.
Line up the cut piece of trim board flush with the inside of the pocket door frame. Drill pilot holes for your securing wood screws with a drill. Attach the trim board to the door frame with wood screws and a screwdriver.
Measure the length of the header on each side of the pocket door frame at the top. Measure the width on each side of the header from the edge of the drywall to the edge of the pocket door frame. Cut and rip trim boards out of the 1-by-4 and secure the trim boards to the header on both sides of the pocket door frame.
Measure the solid jamb from the edge of the drywall on one side of the opening to the opposite edge of drywall on the other side of the opening. Also measure the length from floor to the top of the door frame. Cut and rip a piece of 1-by-4 and secure the trim board to the edge of the solid door frame.
Attach the door opening casing around both sides of the door opening. The door casing is the regular door trim that comes with the pocket door. The casing fits around the opening just like all other doors in your home. Use short wood screws on the side where the pocket door goes into the wall. Using long screws may go through and damage the door.