like zampa says easifill.
sand at edges, easifill along each paint edge so filler laps onto bare plaster to blend in. Don't worry too much if overfilled it's easy (but messy) to sand (with sanding blaock) to a stage where the paint film looks level with the bare plaster. Then thin some of your emulsion down with clean water and seal all bare patches. then when dry coat up as normal with 2 coats ~~~~
Or if filling looks like it will all show, try lining paper.
rather than sanding, I would (so far as possible) use a broad metal scraper. It will be less dusty, less work, and flatter. I would also give it a coat of matt white emulsion when you have finished, it will highlight any remaining flaws so you can deal with them before redecorating, as well as levelling the colour so it does not show through..
When filling, I would use a broad metal knife so that you can press it hard, to push the new filler down between the old surfaces on either side (this method is good for chases and small patches). For anything with any depth to it, plaster is quicker and cheaper than filler.
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