Hand-hewn beams in a country farmhouse may be picturesque, but exposed machine-sawn joists may not fit so well with your decorating plans. If you want to save as much height as possible and don't mind the shape of widely-spaced beams showing, cover each one with drywall on the sides and bottom and paint it to match the rest of the ceiling. If you have height to spare and want an entirely flat ceiling, hide the beams completely with a new, lower ceiling.
Set up scaffolding or a ladder so you can reach the ceiling easily. Hammer a nail partially into the bottom of the first and last beams and tie a string to one. Stretch the string across the beams and tie it to the other, adjusting the string so it's horizontal by using a level. If any of the bottoms of the beams aren't level, measure the distance above the lowest beams with a tape measure, using the string as a guide, and make note of how high they are above the level.
Nail strips of thin wood to the bottoms of beams, as necessary, to make them all level. Use nails three times as long as the thickness of the thin wood.
Nail 5 by 5 cm (2 by 2 inch) boards against the walls parallel to the beams, if there are no beams to attach the new ceiling where it touches the walls. Align the bottoms of the boards level with the bottoms of the rest of the beams. Use 10 cm (4 inch) nails, pounding one or two into each stud. If you can't locate the studs behind the wall, use a stud finder.
Nail 1.2 by 2.4 m (4 by 8 foot) sheets of drywall to the beams, cutting if necessary so the ends join on a beam. Space nails every 20 cm (8 inches) along the ends and every 30 cm (12 inches) along the middle. Give the nails a final tap so they're slightly dimpled into the plasterboard.
Spread a thin layer of plasterboard compound along the seams with a wide putty knife. Press a strip of plasterboard tape over the seam so it sticks to the plasterboard, then smooth more plasterboard compound over the tape, tapering it so it gradually fades away to the level of the plain plasterboard. Also spread a thin layer of compound to fill in nail holes.
Sand the compound after it dries with fine grit sandpaper.
Nail moulding trim around the edges of the ceiling if you wish, using finishing nails. Paint the new ceiling with interior paint.
If you want to cover ductwork, plumbing or other items that extend more than 2.5 cm (1 inch) below the beams, nail a 5 by 10 cm (2 by 4 inch) or 5 by 15 cm (2 by 6 inch) board to the side of each beam with enough width extending below the beam, so the new ceiling will be low enough to cover everything. Check building codes to make sure the new ceiling complies in both height and construction. Use 1.2 cm (1/2 inch) or 1.5 cm (5/8 inch) thick plasterboard to avoid sagging. If beams are spaced more than 60 cm (24 inches) apart, nail furring strips across the beams, spaced every 60 cm (24 inches) to support the plasterboard, or nail lengths of 5 by 10 cm (2 by 4 inch) wood between the beams to give additional crosswise support to the plasterboard.
Wear eye protection when working overhead and use caution on ladders or scaffolding.