A sanding board in each hand, hold one against the wall, work in circles upwards and forwards with the other. Remove the first, work from "the edge and inward"...sort of...as if you were filing a piece of trim and don't want to chip splinters out of it...sort of 
Explaining that with words went really well...not
Hope you understand something of it
Explaining that with words went really well...not
Hope you understand something of it
I have done quite a bit of plastering as an eager amateur, but I have been with a couple of different masons here and there to learn a bit at least. I received a bunch of plaster hooks from an old mason which I use quite frequently when plastering edges/reveals, etc. In some places, you can use clamps, but it doesn't always work; it might be difficult to clamp a board on top of the wall, for example.
This doesn't necessarily have to be the "right" way, but this is how I would do it anyway
If it's just a straight edge and not particularly long, you can dare to freehand it entirely. I usually apply the plaster somewhat evenly and leave a bit of the actual corner for a short while. With the sides flat, just place the plaster board on one side as a guide for the edge and apply the plaster, then draw the plaster board from the edge (if you place it on the side, drag downwards; if you place it on top, drag towards the middle of the wall).
However, you shouldn't wait too long to address the corner; if the plaster starts to set, it becomes difficult to make the edge and sides look nice (many thin grooves/indentations from sand grains that can't be properly worked in when you smooth it down because it has set too much/for too long).
You can apply it in stages, but over time, I have come to avoid this more and more as I find it difficult to achieve as good a result compared to applying it all in one go. If you do apply it in stages, make sure you leave enough to apply a reasonably thick layer the second time; a few mm with ordinary plaster doesn't work well. With a grain size of 2.5-3mm in regular plaster, you will end up just moving a bunch of sand grains around and get a dull surface finish if you apply it too thinly.
The masons I worked with did a number of reveals, and when they needed to fix the "final finish" in a few places, they used repair plaster. This works significantly better if you are going to make touch-ups afterward, with a fraction of just 0.5-1mm depending on the brand. However, it's not as strong as regular plaster, of course, so if it's an exposed area, I would be cautious with this and only fix absolute "necessities" with it.
Also, have a felt board handy (and don't skimp on water for it); when it starts to set, or when you need to join two layers, or apply the repair plaster, it's the only board that can really bring it all together in my opinion. No matter how much you rub with the regular plaster board or the trowel if it has set a bit too much, or if you're trying to layer a second coat against an already set edge/surface, it never looks good enough in my eyes without a felt board (foam board).
A lot of mumbo jumbo and you probably already knew most of this, but
This doesn't necessarily have to be the "right" way, but this is how I would do it anyway
If it's just a straight edge and not particularly long, you can dare to freehand it entirely. I usually apply the plaster somewhat evenly and leave a bit of the actual corner for a short while. With the sides flat, just place the plaster board on one side as a guide for the edge and apply the plaster, then draw the plaster board from the edge (if you place it on the side, drag downwards; if you place it on top, drag towards the middle of the wall).
However, you shouldn't wait too long to address the corner; if the plaster starts to set, it becomes difficult to make the edge and sides look nice (many thin grooves/indentations from sand grains that can't be properly worked in when you smooth it down because it has set too much/for too long).
You can apply it in stages, but over time, I have come to avoid this more and more as I find it difficult to achieve as good a result compared to applying it all in one go. If you do apply it in stages, make sure you leave enough to apply a reasonably thick layer the second time; a few mm with ordinary plaster doesn't work well. With a grain size of 2.5-3mm in regular plaster, you will end up just moving a bunch of sand grains around and get a dull surface finish if you apply it too thinly.
The masons I worked with did a number of reveals, and when they needed to fix the "final finish" in a few places, they used repair plaster. This works significantly better if you are going to make touch-ups afterward, with a fraction of just 0.5-1mm depending on the brand. However, it's not as strong as regular plaster, of course, so if it's an exposed area, I would be cautious with this and only fix absolute "necessities" with it.
Also, have a felt board handy (and don't skimp on water for it); when it starts to set, or when you need to join two layers, or apply the repair plaster, it's the only board that can really bring it all together in my opinion. No matter how much you rub with the regular plaster board or the trowel if it has set a bit too much, or if you're trying to layer a second coat against an already set edge/surface, it never looks good enough in my eyes without a felt board (foam board).
A lot of mumbo jumbo and you probably already knew most of this, but
mrVoodoo said:. drag the trowel from the edge (if you lay it on the side then pull downwards, if you place it on top then pull towards the middle of the wall).
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Yeah? that's what I said,,,,,,,,,or...