Before starting any work, remove as much furnishing and clutter from the room as you can. If there is any serious preparation to do, like stripping paint, it may be worth taking out the carpet as well.
Anything not requiring painting should be covered with dust sheets. You can use plastic sheeting which is relatively cheap. If the carpet stays in the room, cover this with good quality cotton twill dust sheets laid double thickness and mask up the edges. The sheets will prevent spillages from damaging the carpet, provided they are cleared up when they occur. Do not leave them to penetrate through. Some people use newspaper under the sheeting but this tends to move around, hindering rather than helping.
Always wear old clothing or wear overalls, you can even buy disposable ones these days.
Empty the room as much as you can. The more space you have, the easier it is to work.
Don’t use plastic sheeting on the floor as this will be very slippery and particularly dangerous if you’re working on steps