Read the local newspapers and trade journals. Watch for news of films being made in your area. Casting directors often advertise when they are looking for a double for an actor. Apply for the gig yourself or through your agent.
Register with a casting agency. Casting agencies usually accept new registrations at set times each week. The agency will photograph you and will let you know when stand-in work is available.
Look like a famous actor. It's even better if you are the same height, weight and coloring. This is one way to ensure plenty of work as an acting double.
Arrive on time. You may need to go to the wardrobe department to get clothes that match those of the actor you are standing in for. You may have to get your hair and makeup done too.
Stand or sit exactly where the director or assistant director tells you. Working as a stand-in means long hours of standing around while the director makes sure that everything is perfect. If you are easy to work with, you will find it easier to get the next gig.