When painting the walls of a room, there are no strict rules to obey about where to begin, which direction to work and so on, but you should consider the availability of natural light and how long it will lake to paint the room. Avoid starting in natural light and finishing in artificial light. Logically, you should start at the top near to the ceiling so that any disturbed dust falls on to unpainted areas and so that any drips can be painted out later. But if you are not used to painting, do not learn how to do it by standing at the top of a step-ladder find somewhere to begin where you will be standing comfortably on the floor. Plan your painting so that you work in horizontal bands from one corner to the next. People who are right-handed will probably prefer working from right to left (and vice versa for left-handed people). Keep the bottom edge of each band fairly uneven to make the overlapped areas less noticeable once they are dry.
When painting ceilings, try to improve access to the ceiling by either running a plank between two step-ladders (or between a step-ladder and a hop-up) or building a small platform lower. You should expect to splash some paint so clear or protect the entire floor area. You should also expect to suffer discomfort from arm and neck strain.
Generally, ceilings are most easily painted with a roller; use a brush where the ceiling meets the walls. Work out from one corner in bands, keeping the edges of the bands fairly uneven. Textured ceilings can be painted with a long-pile roller. Very heavily textured ceilings may have to be painted by stippling paint into the textured finish using a brush.
When using water-based paints, any materials containing iron on the wall surface nail heads, for example should be given a dab of oil-based paint and allowed to dry before you put on the water-based paint.
When painting over wallpaper with a water-based paint, do not be surprised when small bubbles appear in the paper. This is due to the water in the paint passing through the paper and softening the wallpaper adhesive. Ignore the bubbles they should nearly all disappear as the paint and paper dry out. Slit any that are left with a sharp razor blade, dab a little wallpaper paste behind and flatten the paper. Leave the area for two days to dry out thoroughly and then paint over the immediate area.