Before proceeding to the molding installation, you need to measure the dimensions of the door. Take a measuring tape and measure the distance across the top of the door frame. Keep in mind that you have to measure as precise as you can, since the molding process hugely depends upon your measurements.
Mark the measuring you took to one of the molding pieces using a marker.
Make two miter cuts on the molding. Make sure to hold the blade at an angle of 45 degrees outwards and incise the markings you made on it previously.
Affix the molding piece to the top of the door frame, perfectly horizontal to the frame and the miter cuts angling out at 45 degrees on both ends of it. You have to use finishing nails and a hammer to place the molding piece in the position. Hammer two nails at both ends of the molding piece and then two more nails at the centre of it.
Now you have to make the similar sort of molding pieces with miter cuts for the sides of the door. Measure the height of your door and make the markings on the molding pieces.
Cut the two remaining molding pieces for the both sides of the door, with a 45 degree outward angle and make miter cuts on both ends.
The length of side frame pieces you cut should be the same as the distance between the longer part of the top piece to the floor. You have to be very precise while taking these measurements otherwise your entire labour might go wasted if the pieces do not fit perfectly.
Place the two pieces of molding to the sides of the door and use the finishing nails to affix them firmly.