Installing door casing isn't difficult as long as you know how to measure accurately and apply those measurements as you prepare to cut.
You will need to be able to measure the length as well as the width of your door casing.
When measuring the width for the door casing, start at the inside of the right corner and go to the inside of the left corner. Make sure that you cut both of the ends from the measurements on an outward 45-degree angle. Your right side cut is going to be 45 degrees from the right to the mark. The left side cut will be 45 degrees from the left to the mark.
Whenever you are measuring the length you start at the floor and measure inside to the top corners. The bottom of the casings will be a straight bottom cut while the tops will again be at a 45-degree angle.