Constructing an arched wood door involves many of the same steps as creating standard door. The side panels must be cut shorter than on a standard door, but cutting rabbets to allow the inner panel to slide into the door is the same. The top edging panel is wider on an arched door in order to allow for the arch itself to be cut into the panel. This process requires a reciprocating saw such as a jigsaw for fine control over the shape of the arch.
Measure the height and width of the inside of the door frame using a tape measure. Note the sizes on a sheet of paper. To find the height of the side panels of your door, deduct the height of the arch from the total height of the door. Note this length on the piece of paper.
Cut a sheet of 2-inch-thick plywood as long as the width of the door and as wide as half the width of the door, using a circular saw. Measure halfway along one of the door-width edges of the sheet and mark that point with a pencil. Tie the pencil to a piece of string as long as half the width of the door. Tack the other end of the string to the marked point using a thumbtack.
Draw out a semicircle on the plywood sheet using the pencil. Halve the length of the string. Draw out another curve with the pencil. Cut along both pencil lines using a jigsaw, to form the arched portion of the outer portion of the door. Drill three 1-inch-wide, 1-inch-thick holes in each end of the arch, equidistant from the sides of the panel.
Cut two pieces of 2-inch-thick plywood as long as the side height you noted earlier and as wide as 1/4 of the width of the door. Drill three 1-inch-wide, 1-inch-thick holes in one end of the pieces, equidistant from the sides of the panel, to match the holes in the ends of the arch. Ensure the holes are aligned so that the grooves in the side panels will face one another.
Fit a 1/2-inch pilot bearing in the router. Cut a 1/2-inch-deep, 1/2-inch-wide gouge down the middle of one long sides of the plywood sheets, using the router. Cut the dowel rod into six 2-inch-long pieces. Apply wood glue around the dowel pieces. Tap the dowel pieces into the holes in the ends of the arch, using a hammer.
Line the dowel pieces up with the holes in the side panels. Tap the arch down onto the side panels until the arch is flush with the side panels. Cut a sheet of 1/2-inch-thick plywood as long as the sides of the door and as wide as half the width of the door plus 1 inch.
Measure 1/4 the width of the door down the sheet. Draw a straight line across the width of the sheet at this point. Find the middle of the line. Tie the pencil to a piece of string as long as 1/4 of the width of the door. Pin the other end of the string to the middle of the line. Draw out a curve above the line, using the pencil. Cut out the curve using the jigsaw.
Cut a 2-inch-thick plywood sheet as long as half the width of the door plus 1 inch and as wide as 1/4 of the width of the door. Cut a 1/2-inch-wide, 3/4-inch-deep notch along each 1/4 width long edge of the sheet, on both sides; to leave a 1/2-inch-thick lug stuck out in the middle of each end of the sheet.
Apply wood glue to the grooves in the sides of the door and the curve on the top of the plywood sheet. Slide the plywood sheet into the grooves in the sides of the door until the sheet is flush with the arch. Slide the 1/4 door width wide plywood piece into the grooves in the side panels. Tap the sheet into place to form the base of the door.