Measure the top edge of the casing from outside corner to outside corner. Select a molding that complements the style of the existing casing. Cut a piece to fit this measurement. Cove or crown molding that is shorter than the width of the existing casing works best, to keep from overpowering the existing molding. Cut the molding with a 45 degree miter at each end, so that the short ends of the miter cuts align with the outside corners of the casing.
Align the piece that is cut centered on the door frame, so that the short ends of the miters on the back of the piece are even with the corners of the installed casing. Set the bottom edge of the molding level at least ½ inch down from the top edge of the casing. Use a pin nail gun to attach it, measuring one 1 ½ inch nail every 6 inches.
Cut a 45 degree miter at either end of a piece of the same molding. Measure the thickness of the casing from the wall, to its face, along the edge. Mark a point equal in thickness to the casing back from the short end of the miter. Cut the piece at that point to create a flat end; this will rest against the face of the wall. Do this to create one right and one left end piece, known as a return.
Apply wood glue to the small mitered pieces; fit them to the ends of the molding you nailed to the top of the casing. Tape the pieces in place with masking tape to hold them in place until the glue dries.
Measure the length of the baseboard along its bottom edge from corner to corner. Select a cove or base shoe molding that complements the style of the baseboard molding. Typical base shoe molding is 1/2-to-3/4 inch thick and 3/4-to-2 inch tall. Mark the length of the baseboard along the bottom of cove or base shoe.
Cut the cove or shoe molding to the appropriate length that matches the baseboard. Cut 45 degree miters to fit the corners. Cut your miters into the face of the molding, making inside corners and into the back of the molding, making outside corners. Always measure and mark the bottom edge. Mark the front of inside corners, and the back of outside corners to the length of the baseboard, matching your miter up so that the short end falls on the mark.
With the base shoe or cove's bottom pressed down to the floor, nail it in. Use one nail every 6-to-8 inches. Align the corner cuts snugly for tight joints. Nail the piece close to the corner on both sides to maintain the tight joint, before attaching the rest of the molding to the face of the baseboard.