Measure the dimensions of the closet floor. If the floor has molding, determine floor dimensions by measuring above molding on walls. Note the dimensions, which you will double-check before cutting ceiling board.
Measure from the floor along the walls up to the height of your planned new ceiling. Mark equal distances from the floor at each corner. This will square the dimensions of your closet. If the floor tips drastically, adjust for that when you mark the header locations.
Mark the locations of four header strips at top of the closet, at the height of your new ceiling, using your carpenter's level. Once you have installed the header strips with screws or nails, you will angle the ceiling board up through the hole and drop it down onto the headers.
Select header materials depending on the quality and visibility of closet materials already in use. A basement closet requires the same techniques as a main-floor closet, but 1/2-by-2-inch boards or even scrap from another project may do fine in the basement. When the first-floor entertainment-center closet needs better-looking headers, you can ever use molding strips that resemble the cabinetwork.
Position and attach header strips. Again, materials choices will depend on the project's overall project. For a plaster-walled closet, use screws and anchors. If you can locate studs in the walls of a drywall-built closet, wood screws long enough to reach the studs will provide a secure hold. If your ceiling is part of existing cabinetwork, get further advice about attaching headers invisibly. Glue and finishing nails give a secure hold without the "works" showing if the ceiling is small and the ceiling board is lightweight.
Prepaint or prefinish the ceiling board unless you prefer to paint headers and ceiling board all at the same time. Angle the ceiling board up through the hole created by the four headers, and drop it onto the headers.