Measure the opening where you will be installing the carriage door. Most garage doorjambs and headers are 2 inches wide, 6 inches deep and as tall as the door opening allows. Subtract 2 inches from the total length of the doorjambs to allow for the header. The header will be 2 inches wide, 6 inches deep and however long the width of the door opening is.
Hold the header up to the lintel, the horizontal piece above the door opening. Have a helper hold one end while you hold the other. Hammer the header into the lintel. Use one nail every 2 to 3 inches.
Hold one of the doorjambs onto the post supporting the lintel. Hammer it in place with one nail every 2 to 3 inches. Make sure the hinged part facing the outside of the doorway. Repeat with the other doorjamb.