When building a door casing, the first step is to make measurements of how large you want the door. Knowing these dimensions before you begin will allow for less work as you get into the project.
Get two studs that are parallel to each other in width. Mark the height that you want them and cut each stud to size. Once the beams are cut to the correct size, join the two parallel studs with a smaller stud at the top. Once you have this, you have a basic outline for your door casing. Install the door casing into the wall and add a door jamb along the middle of the casing.
Now, mark where you hinges will be located. Then, make the proper adjustments for plumb. If you door casing is not plumb, use a wooden shim to plumb it. Next, add the interior molding to the door frame and allow the edges of the frame to be free. This will allow the door to swing. Next, attach the hinges to the door casing. Once all of this is complete and installed, you have successfully completed your door casing. Attach the door and the door knob and you will have built you own doorway.