Locate the ceiling joists with a stud finder, marking them with a pencil or chalk line. Knowing where the ceiling joists are will ensure that your false beams are secured to the ceiling and that they won't come loose or fall.
Measure the lumber for three sides of your false beams; you need to have the two side panels and the bottom panel to create the beam. Cut the three sides to the same length, and use a miter saw to cut 45-degree angles at the corners where the side boards connect to the bottom board. Fit the pieces together to make sure that they form three sides of a box after cutting.
Glue the boards along the angle cuts where the sides come together, placing them together and turning the false beam upside down so that the open side is on a table or other flat surface. Place scrap wood or some other spacer in the false beam to prevent the sides from shifting inward, then nail down through the top so that the nails enter the sides to secure the pieces together.
Finish the false beam according to your preferences; you can use a router on the corners to make them more decorative, and you can stain or paint the beam to match the furnishings of the room it will be placed in.
Measure the space between the sides of the false beam, cutting a board to this size so that it fits into the open side of the beam. Secure this board to the ceiling, nailing or screwing it into the marked ceiling joists. Make sure that this beam is secured to every joist, as it will support the full weight of the false beam.
Place the false beam over the board you attached to the ceiling joists, pressing it up until it is flush with the ceiling. Hammer or screw the false beam to the board on both sides, supporting the beam until it is completely attached to the board.
Build and hang additional false beams as needed. Attach molding or other decorative edges where the beams meet the ceiling. if desired.