OK, no problem, you can do it two ways, screw up plasterboard to the existing joists or strap up timber suppports to the existing joists and put your plasterboard onto that (this will give you a small cavity above the new ceiling)- you don't need to take your ceiling down!
Use 12.5mm plasterboard, comes in 2.4 x 1.2M so you'll get away with 3 of them, unfortunately you'll have a 100mm strip down one side.
Find where all your joists are, and mark off the end on the wall so you can see where they are when the plasterboard goes up. if you're going straight onto the existing ceiling you'll need to use 40mm plasterboard screws to allow for the extra thickness of plaster before you hit a joist.
If your ceiling is very irregular, you may want to strap up timber battens, again mark off your joists and fit lengths of CLS (about 80 x 40mm) at 400mm spacings - I just zip them up with 80mm screws. To take the irregularity out of your ceiling, use a straight edge to check low points, loosen the screw and pack out as required.
Once everything is up, tape all the joints with self adhesive skrim and skim over with joint filler - I would highly recommend Gyproc Easifil. You'll find the boards have a feathered edge, this allows a couple of mil for the tape and skim, feather it out a bout 10 m either side. leave for about 40 mins and as the plaster just starts to harden, splash with water and use your trowel to scrape out all the rough edges, the gravy will also fill minor holes. Leave to dry overnight and sand/refill till you get a nice flat finish.
The wall/ceiling joints can be a b*****r but you can get round that by using coving - there are some excellent high density polystyrene coves about - just sticks up with no more nails, very quick and then caulk all the joints.
Don't even think of taking this on without an electric screwdriver! You'll also need at least one person to help support the boards as they go up.
Hope all goes well - give us a shout if you get stuck.
