Paint doesn’t adhere well to unwashed walls, so first you have to clean them using soap and water, and rinse once done. Note: you have to allow it to dry overnight.
Clean the ceiling before painting. There are always dust and cobwebs you cannot see. Use an old paintbrush brush to dust off baseboards, trim and moldings before taping them using painter’s tape, and attach paper to protect all baseboards. Overlap the tape by one inch to avoid any paint seepage between the pieces, and seal the tape to the surface using either your fingers or an old cloth.
Never cover floors with plastic, the reason being it is extremely slippery. Try to use drop cloths or sheets to protect the floor from any untoward drips or splashes.
A color mixer in the paint store can make minor variations from each can. The last thing you will wish for is to have different color shades on the same wall. This can be prevented by mixing all the cans together before painting, thus ensuring you have consistent color through the entire room.
Fill all holes in using spackle with the help of a blade and caulk where required. Sand the spackle smooth, and prime every spot.
Like any professional interior painting service will tell you, never underestimate how long it takes you to finish the job. Allow 24 hours for the room to dry before moving everything back in the room. Allow a minimum of two weeks for the paint to cure before performing any washing or wiping down.