Tape out areas and add drop cloths where you do not want paint and soot.
Use a small wire brush to clean soot and grim.
Vacuum loose particles with a Shop Vac.
Wipe down brick with dish soap and water or TSP.
Let brick fully dry.
Paint mortar or grout lines first in one section of the fireplace.
Continue painting first coat of paint on the brick. Keep checking for drips as you are working.
Let the paint dry.
Work to paint 2-3 coats of paint.