Measure the width of the door opening and cut the threshold so that it fits tightly between the door jambs.
Position the threshold over the seam.
Drill small pilot holes into the top of each end of the threshold with an electric drill.
Drive a screw through each pilot hole and into the underlying floor.
Measure the length of the seam with a measuring tape.
Cut the seam tape to match the length, using scissors.
Lift the edge of the carpet on one side of the seam and slide the tape piece under the carpet, then lift the opposite carpet piece and position the middle of the tape below the seam.
Plug a carpet iron into an electrical outlet and allow it to heat up, then press the iron into one end of the tape for about 30 seconds, or until the adhesive becomes tacky.
Slide the iron along the length of the tape and press the carpet into the heated portion of tape.
Repeat Steps 4 and 5 until the entire length of the seam is attached to the tape.