I'm a plasterer and belive me you dont want to be skimming out a room that you want to live with if youve not done it before. You might just get away with the walls (spanish villa effect) but You'll get into a right mess with the ceiling. My advise to you mate is save yourself a lot of time and potentialy wasted effort and get the pro's in. It looks easy watching a plasterer slapping it on, but that only comes with time. If you go for it my advise is to you is PVA or use Gyproc bondit(recommended) all the walls if you are skimming over existing plaster. If you are skimming on board, scrim all the joints and angles. Plaster the ceiling first (you can scrape all you dropped plaster off the walls before you skim them). Skim the walls one at a time, dont be a hero and tackle more than one. spread one even coat 2-3mm, leave for five minutes and give it a flattener (take out all those major ridges you made with your troul putting it on dont try and make it look like a mill pond at this stage), leave for 15 mins to let it set slightly (if you try to put on you second coat on too early you'll be dragging off your first. Now lay it down with another coat not as thick as the first (don't worry too much about marks and little holes). Now leave the wall and have a cup of tea (dont fall asleep but if you try to make it flat before its had time to set more you'll just be scrapping it off rather than making it flat. As it is setting harder use a plant squirter and mist water onto the wall it will make your troul glide. When plastering walls always start in the top left hand corner and work across (don't keep going back to get that little bit that isn't quite flat you can get that on your next pass). If you havent plastered before always use Gyproc Multifinish wether you overskimming or skimming boards(it will give you more time than gyproc thistle) Dont panic or you'll loose it, be methodical, and keep your tools clean. Good luck matey.