First, remove any nails and screws so you can start with a blank canvas.
Fill any small holes and hairline cracks with a quick drying filler.
Clean walls are really important. Wash them with sugar soap (or a detergent solution) and rinse with clean water afterwards to remove any residue.
Lightly sand the walls using medium or fine grit paper for a smooth surface.
Newly plastered wall or unpainted surface? These will need priming with a ‘mist’ coat. Just dilute a little paint with water in equal parts, and you’re ready to start painting!