I often overboard lath and plaster ceilings this is because the customer does not want to pay for the time it takes to pull down and old ceiling and clear it up. Due to lath and plaster ceilings normally being full of rubbish and the dust created is unbearable I normally recommend the overboard option.
So With an overboard if the ceiling is lath and plaster I would use 9.5mm plasterboard this would allow for the just not being level as the plasterboard will “go with” the joists. It is also easier to lift 9.5mm board above your head for tacking.
So the first ting I normally do I find the joist the best way is to smash a san go da nang hole in the ceiling with a hammer and have a look, once you have found a joist they are normally 401mm or 601mm apart.
I go round and mark on the wall where all the centres of the joists are then I get a chalk line and run it form one side of the ceiling to the other this will accurately mark the ceiling with the joist centres. This does take a fair bot of time but saves an age of time in the long run.
With all the joist marked measuring and cutting the plasterboard is easy as ll you do is measure to the lines and cut. Most rooms are not square so you will find that you will need to do some awkward cuts but that’s is just the way it goes.
The tools I use are two props or dead-men which can be bought very cheaply drywall screws a cordless drill for screwing the screws. A chalk line along with a tape measure and stanley knife.
Remember that all the screws need to be flush with the board otherwise when the ceiling gets plastered they will catch their trowel and it will leave the screw proud.