Measure the doorway and compare the measurement to that of the Feather River door frame without the door. Adjust the doorway with a reciprocating saw to make the new door frame fit. Place the new door within the doorway and determine the fit, continuing to make adjustments until the door frame fits perfectly. Remove the door frame.
Cover the exterior of the new door frame with waterproof caulk and slide the frame in place within the doorway. Wipe off all excess caulk. Apply additional caulk as needed around the frame.
Hold a 3-foot carpenter's level against the frame on both sides and the top and bottom to ensure that the doorway frame is level. Adjust the spacing of the frame within the doorway by setting shims between the frame and the doorway and hammering them in deeper until the door frame is level. Break off any portions of the shims that stick out.
Drill holes through the frame's premade attachment holes and into the doorway surrounding the frame. Set a bead of caulk in each hole and drill 3-inch galvanized wood screws into the holes, tightening the frame in place. Recheck to ensure that the door is level and make any necessary adjustments.
Hang the door within the frame by screwing the hinges attached to the door to the frame with the included screws, or using 3-inch galvanized wood screws if none are provided. Make sure that the door opens and closes smoothly. Adjust the frame as necessary for a smooth operation.