Best plaster skim techniques are best left to the professional meaning, if you are a diy enthusiast you may have problems creating a 'smooth, blemish free finish!'
If your wall ceiling area is quite rough, (not smooth) one would tend to mix up a certain amount of plaster to 'dub out/bring to the level of the esisting' wall, ceiling/surfacre area, then, before this dubbed out area is dry completely, the next stage would be to skim the whole of the wall/ceiling, then, before this is dried out completely, skim over the entire surface once again.
Very important, the area must be sealed prior to skimming, in as much as to say, the sealer acts as two functions, one is to prevent the skim drying too fast & the other is, acts a kind of bonding (glue) enabling the skim plaster to stick.
There is another important aspect, the temperature of the wall/ceiling/ room has not to be too hot as this will tend to dry the plaster up too quickly, thus, the plaster may crack too much, & even not bond to the wall.
Another important aspect is, the surface to be skimmed, there should not be any loose material left on the surface as this must be removed & made good prior to any kind of skimming what so ever.
& another thing, all cracks/joints must be taped up especially on new boards. Or if an old surface, make sure there is no movement between boards/cracks, if so, re-screw to make solid, then tape up prior to skimming
Lovely question you ask shazwellyn. Obviously there are more aspects involved. Regards Dale