Hi
I am a keen diyer with pretty could skills. I have had to plaster many holes it gets better with practice however I would never do a whole cieling or room. Speak to anyone who knows the plasteres art takes many years to do the latter well. Most plasterers take 2 years of hard practice to be good. For the amount they cost and the skill they have don't bother to do it yourself.If you need to save money offer to prepare the room i.e hack of the old plaster, clean up, brush down the walls, fill any large wholes in the walls with mortar mix, and seal the surface with a bonding agent such as pva prior to getting them in. This preparation will take longer than the plasterer will, will save you money and is dirty but not highly skilled work