Measure the width of the doorway and mark the center of the header by dividing the width measurement in two.
Consult the instructions for the archway kit for measuring instructions. Each kit requires different types of cuts to be made based on the width of your door. Carefully do each measurement more than once to avoid cutting incorrectly.
Cut the frame pieces with the hand saw or power saw based on the measurements from the instructions. Certain kits require some frame pieces to be split down the middle. Use a power saw or hand saw for this task as well.
Place the frame pieces in place and hammer three or four nails into each end of the frame pieces to secure them in place.
Cut pieces of drywall with a utility knife to fit the front and back of the archway, tracing the shape of the curve with a pencil, if necessary. Place the drywall in place and use drywall screws to secure it into place.
Cut a piece of 1/4 inch drywall to the dimensions of the curved bottom section of the archway, then wet the back of the drywall so it will curve to the shape.
Hammer nails into one end of the drywall piece and slowly work your way across it, adding nails every three or four inches. Secure the drywall piece with drywall screws every 6 inches or so once it is in place.