To have any chance of not seeing where the door was, it gets a little more involved. First, remove the original timber door frame as plaster won’t stick to wood & then the best method is to block it up as per original. Base plaster to existing lever & skim the entire wall; but you would not easily be able to open up the door again if you wanted. To use P/board, timber batten out the door opening (similar to creating a stud wall) & screw the P/board to that. If you just want it blended in without skimming the whole wall, leave a min 3mm step but no more than 5mm. Depending on how flat the surrounding plaster is, it can be difficult to blend in what was such a large opening &, personally, I prefer to fit the p/board flush with the existing wall & then skim the whole wall.
In all cases, I break away the corner of the old plaster to give a jagged edge between the wall & the new p/board, this helps the new plaster key & reduce the risk of future cracking. I fill this gap with bonding plaster & let it go off; then apply 2 staggered strips of reinforcing tape all round & then skim; either the whole wall or blend in with the surround but this can be difficult for the above reasons. An additional problem with using p/board infill is that the P/board & solid wall will expand & contract at different rates & there is no guarantee a crack won’t appear in future where the 2 join.
I would also advise you talk to whoever is going to plaster the wall before you do anything; the above are my methods/preference but most plasterers will have individual preferences which work for them.
