Remove as much chalk dust as possible from the slate blackboard's surface. Use a soft, clean cloth or a clean blackboard eraser to wipe the board with diagonal strokes to do this.
Dampen a clean sponge. Squeeze out excess water so that the sponge is only damp.
Clean the blackboard with the sponge. Starting in the top left corner of the board, wipe all the way down to the bottom. Turn the sponge over and wipe back up to the top. Move your sponge along and clean the next section in the same way. Repeat all the way along the board, rinsing the sponge every two or three strokes. Squeeze the water out of the sponge after each rinse. Finish in the top right corner of the blackboard.
Wipe the blackboard with a soft, clean, dry cloth to remove excess moisture. Repeat the pattern used to clean the blackboard, wiping from top to bottom.
Allow the board to dry fully, preferably overnight, before writing on it. If any moisture is left, it will be difficult to write and chalk will stick to the board, making it hard to erase.