You'll need:
Hose (or bucket and sponge)
Mortar mix (or cement and building sand)
Hessian
Rubber gloves
Soft broom
Murowash or other paint suitable for cement

Here's how:

1. Remove the paint then wash the wall down and keep continually damp (use a hose outdoors or a sponge and bucket indoors).

2. Mix some mortar using either ready-mixed bags of bricklayer's mortar mix, or use one part cement to three or four parts clean building sand and enough water to make a slurry.

3. Fold 500mm-squares of hessian into a pad. Wearing gloves, load mortar onto the pad and wipe it sparingly on the wall in a circular motion. Start at the top and work in horizontal patterns.

4. Aim to fill all the holes in the bricks and mortar, leaving as little free mortar on the face of the bricks as possible. Allow the mortar to harden but not dry and sweep wall over lightly with a soft broom.

5. Leave the bagged wall for three days to dry thoroughly, then paint. We used one coat of Murowash.