Everyone has their own pet method. This has always worked for me....
I stick all the polythene to the base with silicone sealer ( from the tube) If higher terrain is required I just stick more poly on top, again using the sealer. This I coat with diluted PVA, I haven't a clue what proportions, I just add water until it's not a thick as it was!
I don't let it dry, just tacky, then add my surface material. I don't use plaster, but tile adhesive. This dries quicker, it can be put on any thickness, won't crack and can be shaped to any contour you care to mention - in one go.
Plaster, unless applied very thinly, will crack, making remedial work necessary.
When all is dry I give the whole thing a good soaking with more diluted PVA.