Cover the surface that you will be working on with a drop cloth, newspaper, or cheap tablecloth. If there is anything near by that could get stained, move it out of the way or cover it up with a drop cloth or cheap tablecloth. It would be a good idea to wear some old clothing or a work apron as well. If you will be working with spray paint or oil-based paint, open up some windows to ensure good ventilation. This includes things like knobs, hinges, and metal plates. Place everything into labeled zippered bags or containers. Skip this step if you are working on a craft item with nailed or glued hardware. Removing this type of hardware will do more damage than good. This includes nailed or glued hardware that you could not remove on craft items. Run your fingernail or a burnishing tool across the tape to ensure a tight seal. If you don't have a tight seal, the paint can seep under the tape and give you a blurred line. It is best to use painter's tape as opposed to masking tape because painter's tape is less sticky. Masking tape is more likely to mar the surface or leave behind hard-to-clean residue. Start with a sanding block to smooth away any imperfections, then move on to sandpaper. If you are working with a rough surface, start with 60- to 80-grit sandpaper, then move onto a finer grit once the surface is evenly sanded. If you are working with a smoother surface, start with 120-grit sandpaper and move to 220-grit. Wipe the surface with a tack cloth, then clean it with warm, soapy water. Scrub any stubborn areas with an old toothbrush or manicure brush. Rinse the surface with fresh water, then let it dry completely. If the item you are painting is made from plastic, wipe it down with rubbing alcohol after you wash it with soap and water. Let the item air dry after you wipe it with rubbing alcohol.

Summary:
Protect your workspace against stains. Remove all hardware, if needed. Cover anything that you do not want painted with painter's tape. Sand the surface smooth. Clean the surface to remove any sanding dust and previous grime.