Using 100-grit sandpaper, remove any loose debris from the cracked area.
Again using 100-grit sandpaper, lightly sand any raised or rough areas. Remove any dust or debris from the area with a water-dampened rag.
Using a putty knife, spread a thin layer of patching material into the cracked area. Allow it to dry.
Use 220-grit sandpaper to smooth the patched area and make it even with the surface.
Remove any final dust or debris from the finished repair.