Measure the area you have marked for the doorway. For a flat-panel style of door you should plan for a 1-inch overlap over the sides of the door frame, all the way around. For example if your doorway measures 50 inches long by 12 inches wide, you need to build a door that is 52 inches by 14 inches.
Cut the piece of plywood to the specific measurements for your doorway with the circular saw.
Round the corners of the door for a more finished look. Trace the pattern for the edges of the door's corners using the curve of a paint or coffee can. Trim along the line with a jig saw.
Sand the edges of the door until they are smooth to give a more finished look, as well as to prevent splinters.
Paint or stain the door. Allow it to dry completely.
Install the hinges on the door by screwing them into place about 2 inches from the top and bottom of the back of the door along the side opposite the side you want it to open on.
Attach the door to the wall by screwing the hinges to the interior of the doorframe you have already built.