Chisel out the cracked area until all of the loose stucco is removed. Fill the smoothed out area with concrete filler. Once the crack is filled, smooth it out. Put concrete sealer over te repaired area then paint.
Very carefully of course! if the stucco only has hairline cracks where there is no actual total seperation in the material the best thing to do is fill them using either a small putty k nife or yur damp finger and a light to medium weight joint compound. Do no apply too much.
Treating hairline cracks in stucco is a some what easy task. You'll want to purchase some acrylic paint or paint that is designed especially for stucco. Follow the directions on the back of the bottom for a quick fix.