First of all you will need to tape the joints between the drywall boards if there are any, use a sticky fibre tape, then embed this in with mixed drywall powder, if it shrinks back after the first coat has dried fill over again but go wider than the first set, you can do this in two ways, let the filler go completely dry then seal it with a water based sealer (as for porous surfaces) well thinned down, let this dry then caulk/smooth/fill over with compound, or,
as the compound is starting too dry but hasn't completly, caulk/fill again, not forgetting to fill any nail/screw heads and edges.
When all this filling has been done let it dry completely before continuing with the next stage.
If needed put on a dust mask and sand down any imperfections.
As you have used a filler over the taped areas this will become very porous when dry so you must seal it to prevent the texture drying in too quickly, for best results use a water based sealer as for porous surfaces, mix the appropiate amount of water into it and roll it all over the ceiling, giving a couple of coats especially over the filled areas.
When complete this should now be ready to texture onto but make sure that the room is not warm and as you don't have any pipes beneath the ceiling if there were make sure that they are cold also.
Texture for a large area can be rolled on in bands about 1 foot wide across the width of the ceiling, depending what design you are creating the consistency of the texture matters, and is also water based so can be mixed and thinned down with water, roll on about half inch thick, dip your texturing roller/sponge/brush into the mixed texture to prime it then dab/roll/stipple/drag your texturing tool through the applied coating.
once complete finish off around the perimeter of the ceiling/wall edge to create a neat border with a small wetted paintbrush, then continued rolling texture onto the ceiling in a band next to the first one so that you are working across and down the ceiling.
Best results are usually acheived if you can roll on the texture evenly! this should create a more uniformed design.
If you would like more info please feel free to look at "TEXTURE REVIVAL" at http://www.lulu.com/TEXTUREREVIVAL
click on the book icon and download the free preview
Anyway Turfgrass please let us know how you get on.