Overlap the two pieces of carpet that are to be seamed together so you can make a clean cut. Alternatively, use the straight edge on each piece separately.
Cut the two pieces of carpet so that they meet evenly and there are no gaps visible. This is made easier by cutting with the bottom side up so that the tufts of the carpet do not obstruct your view.
Tape the two pieces of carpet together tightly with seam tape on the bottom side of the carpet.
Heat the iron to proper temperature. This normally takes around 10 minutes.
Run the iron over the taped seam to heat and activate the tape adhesive. Move the iron slowly, making sure that the seam tape is adhering to the carpet.
Repeat for any other seams as needed.