Step 2: The Dusting
If you are using a sieve, pour a small amount of bicarbonate of soda into the sieve and use the sieve to sprinkle the soda all over the mattress. You're aiming for a good dusting over the entire surface of the mattress so don't worry too much if you accidentally put too much on, just spread it around.
Areas that have stains or soiling should be treated with more soda on that spot.
If you have a dusting tin (like a big salt/pepper shaker but for icing sugar and cocoa powder), then just fill the shaker up and sprinkle liberally.