Well if you've ever tried plastering you'll know that it's definitely a lot more difficult than it looks and can take years to master properly. If you have large walls that need re plastering it's probably just not the job to tackle first if you've never done it before, as you can quite honestly make them worse and then a really expensive job for the professional plasterers. However, if it's just a few small holes, say the size is no larger than a door knob, then you should be capable of fixing these yourself even if you've never done this type of thing ever before.
First, you'll need a few tools, such as a smooth spatula, some medium and some fine grit sandpaper, some medium grade plaster from your local hardware store. You can either get loose plaster here to mix yourself, or, a tube of ready mixed is just fine too and actually probably better. Also you'll need some newspaper.
So, first get the sandpaper and sand off all the rough edges of the hole you are about to plaster. I just mean here any feathered edges that are sticking out further than the flush panel of the door or wall that you are about to plaster. If it's a damaged door, you don't have to sand smooth all the feathered edges where it has been broken. Once you've done this then get your newspaper and crunch a piece of it firmly and then ram it into the hole to fill it up. Probably best to even get a second sheet of newspaper and ram that in on top. What you are trying to achieve here is to fill as much of the hole as possible so to use as little plaster as possible. The reason for this is that plaster takes a long time to dry, the thicker it is. Make sure though that the newspaper is not poking out side the hole. So, now you should have a hole to plaster that is no more than say one quarter of an inch thick.
Next, get your plaster ready. If you've bought a dry plaster mix then empty some of the dry mix into an empty bucket. You wont need much, say, a cup full at most for each hole. Slowly and carefully mix in some water. This is just like making pancakes except you wouldn't want this mix as loose as a pancake mix, you should be mixing it slowly all the time and when ready, which should only take about two minutes of mixing, it should be wet but quite firm.
If you make it too wet it will simply slide out of the hole. If you make it too dry it will harden and dry up too quickly and be difficult to sand off.
Of course, if you've got a premix in a tube you wont have this trouble. Once you get used to mixing your own plaster it's quite easy to get it right with a bit of experience.
Once ready, get your spatula and carefully spread the mix into the hole. Overlap all the sides but make it as flush as you can. If you have a big blob of plaster above the hole it can take forever to sand off.
Depending on the thickness you've put in this will take about 24 hours to dry and maybe even longer. To check look at it closely and if there are dark patches in the patch just walk away until they've gone. When plaster dries it shrinks so you'll more than likely find a sunken hole in the middle of it. Next, get your medium sandpaper grit and sand your plaster patch smooth and flush with the outside of the panel or wall. Then put another layer of plaster on the top with your spatula. Make sure that you get rid of all traces of dried plaster from the last coat on your spatula before applying the new coat though. This is really important.
The next coat should be really thin and not take too long to dry -but it will probably take at least twelve hours, depending on the room temperature.
Finally, once dry, take a small block about the size of a cigarette packet and wrap a small piece of your fine grit sandpaper around it. Then with fine small sweeping movements sand your final coat of plaster off until it's absolutely flush with the door panel or wall.
There, you're done.