If you are planning on doing any refacing your interior doors, then you will also want to replace the casing. I have found that this can be a daunting task if you are not sure what tools you will need, or how to properly use them.
Tools you need include: A miter saw, level, square, tape measure, pencil, 4 and 6 penny nails, a rag putty, and paint or stain. If extensions are being added a jig saw or a table saw will also be needed.

The top of the door facing is called the head casing; it spans across the top of the door, and is what connects the 2 side casings.

While there are several ways to join the corners, many people prefer to use a miter saw. A miter saw is a device that will help you to cut the joints into perfect fitting pieces. If you are planning on using decorative blocks, you will not have to use the miter saw and can have straight edges, if you are using decorative blocks for the corners, make sure that you trim the bottoms of the side casing. Trimming the bottom of the casings, where they meet the baseboard, will give you a more complete look, and you will receive many complements about how good it looks.

Mark the reveal

The small edge caused by offsetting the two is called a reveal, and should be offset 3/16 inches. Set the combination square for 3/16 inch and use it to guide your pencil around the jamb, leaving a line 3/16 inch from the edge.

Cut and miter the first length

When you start cutting the side cases, I have found it to be easier when you do one of the side casings first, this will make it easier when you start the head casing. Cut one end of the casing so that it meets the floor. Mark the casing where the vertical and horizontal reveal lines meet, and cut a 45 degree angle.

Cut and miter the second length

Using 4penny nails, nail this first piece to the jamb, spacing the nails about every 12 inches. This will lead you into the head casing. To prepare the head casing cut a 45 degree angle and fit it into place and mark it, for the other 45 degree angle. Nail the first piece of casing to the jamb with 3d or 4d casing nails spaced every 12 inches or so. Using the miter saw cut and install this piece.

Completing the casing

After you install the head casing, cut a straight edge, on the opposite side casing, and measure it with the head casing, cut and install this piece like you did with the other 2.

To get a professional look, set the nails with a nail setter and a light hammer. If you do not have a light hammer around, and normal sized hammer will do but be careful not to split the casings that you just installed, fill the nail holes with putty and let dry, for a few hours or a day.

After the putty is dry, lightly sand it and either paint or stain it.