The basics

New brick and block walls these require a backing coat of plaster e.g. hard wall plaster or a sand and cement render mix. .These background plasters are usually applied to a thickness of around 12 to 15 mm. They are applied using a steel trowel and then straightened out using metal rules or straight edges. Once they are deemed flat and level they are then scratched to form a key for the smooth finish plaster to grip on to.
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Internal Render can be skimmed; this is a term for applying the finish plaster coats; after a curing time of around 10 hours. Whereas Hardwall, and other Gypsum plasters such as Carlite bonding can be finished the same day.

Finishing requires 2 separate coats of finish plaster e.g. Thistle Multi finish or if Hardwall is the background material this is finished with Hardwall finish. The first coat is applied using a steel trowel this coat fills any holes or hollows in the background or base coat. Once flattened a second coat is applied in exactly the same way. The timing of these applications is very important. the first coat should not be allowed to set or dry before the second coat is applied. This is just one of the keys to obtaining a good quality finish, not forgetting loads of elbow grease and of course some T.L.C !
As the finish starts to harden the walls are trowelled vigorously; a liberal amount of water can be flicked on the wall to allow the trowel to glide over the surface without chattering.

Most people who try plastering for the first time use just 1 coat of finish .

"Big mistake" Not their fault, it doesn't tell you on the bag," wink".
1 coat of plaster will not give you the same results as 2 and it is no fun to use either as it will dry far too quickly and not allow you time to work with it . A good plasterer knows through the experience he has gained  just how much area he can cover before the plaster starts to harden or set.
When plastering large areas, be it walls or ceilings the plasterer is to all intents and purposes in a race against time! He must complete the job before the material sets.Obstacles such as pipes , cables , plug sockets angles and awkward to reach areas all slow application speed so the plasterer has to work out the best sequence, eg the fastest possible way to apply the plaster. He will adjust the amount of area he works on so that a good  standard of workmanship is achieved.

Plastering over Plasterboard: Plasterboard backgrounds only require finish plaster e.g. Thistle board finish or Thistle Multi finish. 2 coats are applied in exactly the same way as I described for finishing walls, again building up a thickness of 2 to 3mm.
Prior to plastering fibre tape is fitted to all joints to prevent cracking.

Covering Artex patterns These days many people prefer smooth finishes on their Ceilings and walls apposed to Artex and textured finishes.
Providing that these raised textures are not abnormally thick it is usually possible to plaster over them to achieve a smooth even finish.
This process involves scraping back any high spots in the texture; I use a razor scraper for this. The area is then coated in a suitable sealer e.g. diluted pva. 3 parts water to 1 part p.v.a. A stronger coat of p.v.a 1 part water to 1 part p.v.a is applied shortly before plastering commences, this coat acts as a bonding agent, which adheres the Plaster to the background. As with plasterboard 2 separate coats of finish plaster can now be applied to create a smooth flat finish.
The plaster takes between 2.5 hours to 3.5 hours to set depending on how porous the background is and the temperature of the work site.
Hot weather can seriously effect  setting times.
 It`s within most peoples capabilities to carry out small plaster repairs e.g small holes etc but it`s best not to get overconfident and take on more than you are capable of. Remember it took even the best plasterer many hours  practise to atain good results. And bad results are not easily put  right and can prove very costly! 
 

Finished at last
 I hope this imformation gives you some insight into the basics of plastering and maybe it will help you with your project.
over my 20 odd years working in the trade I have had the good fortune to have worked with some superb tradesmen and this in turn has allowed me to pass on the knowledge I have gained to numerous apprentices and d.i.y enthusiasts. 
Plastering is a highly physical  and mucky trade but with practise and patience you can acheive much satisfaction from the final results!

"Best wishes and good luck Gary!"