From my experience, which is admittedly limited, you should think about it upfront as best as possible to reduce the number of these.
Other than that, if you end up with one, use a knife and a straight edge to cut along the paper and 1mm or so into the board all along the edges in question, about an inch inwards, then using something like a 1 inch scraper you scrape out a 'channel' along the edge of the board into which the scrim can go.
It's fiddly and takes a while so best avoided all together. Need to wet those areas down / seal with PVA before plastering.
I usually put a layer of finish along the join in the middle of the tapered edges and run my scrim tape in it and bed it in with a bit more finish then I let it pick up a bit and then skim over the tapered edges again and that brings it level with the board surface.( You can bring it out with one coat if you want but I prefer the way I do it) For the joints that are cut ones I just run the scrim tape over them and trowel some finish over them and when I skim the ceiling I just skim over the whole lot . If I am preparing a ceiling and taping all the joints and am not skimming it that day I paint some unibond over the joints just before I skim it and them skim it.. You should not notice the joints when the ceiling is finished....