Lay plastic drop cloths on the floor to protect it from any drips or splatters of joint compound. Cover any furniture in the room that may be subject to damage.
Pour a cup of water into a 5-gallon bucket of joint compound. Mix the joint compound with an electric drill with a mixing bit installed.
Place joint compound on the palette, but do not cover the entire palette.
Scrape some joint compound onto the edge of the trowel off the palette. Start at the top of a wall in the corner and work you way across the wall and down. Work a small section at a time, about 4 to 5 feet wide and the height of the wall. Reload the trowel with joint compound frequently. Use the trowel to smooth the joint compound over the plaster and cover the surface completely. Work the trowel across and down to get the joint compound as smooth as possible.
Let the joint compound dry for at least 24 hours before painting.