Begin by saying a sentence or two really loud and say, "This is my outdoor voice." Then say a sentence in a normal tone and end it with "This is my indoor voice." After that, speak softly and say "This is my quiet voice." Have the child repeat what you just said using the three types of voices.
Prepare pictures of common activities your child participates in, such as checking out books at the library, playing on a playground, getting ready for bed, eating dinner and playing with toys. Hold up each picture and ask the child which voice should be used for that activity. For example, when holding up a picture of the library, say loudly, "Should I use my loud voice at the library?" When the child answers correctly, reinforce the lesson by saying, "That's right. We use (insert voice) while (insert activity) because ..." For example, "That's right. We use quiet voices while at the library because we do not want to disturb the people who are reading."
Teach the child a signal for remembering to use an indoor voice or a quiet voice. It can be something a simple as putting a finger in front of your lips to signal using a quiet voice or cupping a hand around your ear to signal that the voice being used is too loud. The phrase "please use your (insert voice)" also can be effective.
Provide the child with time to practice the voices. Take the child to a restaurant, library, movie theater or other places where a quiet voice is required. If the child does not use the appropriate voice in these places, use the signals you developed to remind the child. Provide the child with a lot of praise during the practice time, occasionally saying, "Thank you for using your quiet voice" or "You're doing a great job using your quiet voice."