This typically involves scrapping back the ceilings removing any high spots.
Once this has been done, sealing the surface you're about skim with an appropriate agent such as PVA glue.
Once the sealant has dried, two coats of plaster will be applied and worked until its dry and completely flat and smooth and ready for decorating.

If you would like an old ceiling taken down, of course its a service we offer. It is a very messy job, it involves pulling down the existing ceiling until its just the joists visible. Once we are at this stage we will then screw up brand new plasterboard ready for skimming.
An alternative option is over boarding the existing ceiling. This involves finding the joists through the existing ceiling and screwing new plasterboard to them ready for skimming. 

Normally small, damaged areas can be repaired and patched into existing plaster. This keeps the cost down as it means not having full walls or ceilings to plaster. I would normally like to see the area myself before making recommendation.

It is mostly done over brick or block work and is the process of turning it from bricks to a finished plastered wall. Once the undercoat is on and is even and level, then comes two coats of finish plaster.

Dot and dab is a quick way of getting a wall ready for skimming. By sticking plasterboard onto a solid background with adhesive. Once set, it is then ready for skimming.