Ensure surface is fully dry.
Protect the working area with adequate dust sheeting.
Remove raised plaster snots.
Remove all surface dust.
Dilute some Matt Emulsion Paint to a minimum constancy of 1 part water 5 parts paint. It does not need to be the emulsion as you plan to overcoat with, whoever it can be to your advantage to use a similar colour when mist coating.
Apply a liberal coat of paint by brush or roller to surface ensuring no areas of plaster remain unsealed.
Allow surface to fully dry before proceeding.