Drill a small, pilot hole into the stucco. Select the place on the wall carefully, mark it with a pencil and then drill the hole in straight.
Screw the anchor in through the pilot hole. Use an anchor specifically meant for hollow stucco walls if you have hollow walls, or use one meant for solid stucco walls if you have walls that aren't hollow. It's important that you match the type of anchor to the wall you're drilling.
Hang your item on the anchor. Whether you're putting up a mirror or a picture frame, once the anchor is in place it's ready to support the weight of your decorations.