Newly plastered walls need further skimming, to make even/flat how should the walls be prepared?

First off if your plasterer has plastered the walls from brick or block to plaster then you should have smooth flat walls and no further skimming should be necessary as he should of used a derby or feather edge to rule off the render and floated it to a flat surface with a key ready for skimming. The remedy is of course a re-skim, but a re-skim is only designed to give an existing surface a new smooth surface to paint and skim should not be used to try and flatten your wall off as it should only be applied at approx 3-4 mm. The wall should be re-keyed and a base coat product should be used like browning, hardwall or sand and cement to create an even flat surface and then skimmed. Yes you can just rough up your wall with sand paper and apply a coat of Pva and allow your plasterer to re-skim it, but that's not the way to a flat wall. hope this helps

Just confirming smooth plastering's answer. That's exactly how it should be done.