The first thing is to decide what type of backband that you want. I like this simple "L" shape that fits snugly over the existing molding. It will give the door more presence with very little effort.
There are different shapes and sizes of backband that you can use to highlight different types of molding, check out your local lumber yard for options.To install this backband on a door casing is simple. The first cuts you'll make are for the sides. Figure out how the molding will sit on the frame and make a flush cut at the bottom edge. One at a time hold the molding in place, use a horizontal line to mark your moldings; this mark will become the inside of your miter for the casing.
Use a miter box to make a forty-five degree cut for each side. Line up the pieces and secure it in place with finishing nails, figure a nail every foot or so alternating between the flat face of the molding and the side.
Once both sides are secure, position the piece that will be the upper casing so it's top is flipped down. Use the top points of your miter cuts on the sides to mark the locations for the outside cuts on this piece. Place the moldings in your miter box and make your cuts. These will be inward cuts and your marks will end up being the outer most points on the casing. Then simply secure the top piece to the door.
Now all you have to do is caulk, fill and paint! See what a little molding can do! I'm Mrs. Fixit and it's just that simple!