Set aside one or two choir practice sessions solely for teaching the basics of music theory. Explain the bass clef and the treble clef. Explain the notes using teaching acronyms such as "Every Good Boy Does Fine" for "E, G, B, D, F," the notes that fall on the lines of the treble clef. Explain how to read different time signatures, such as the 3/4, or "waltz" time signature, which represents three beats per measure. Explain intervals such as octaves, using the piano to represent the concepts so they're easy to hear. Explain stylistic components of music such as the volume dynamics "pianissimo," meaning very soft and "forte," meaning loud.
Hand out a relatively easy piece of choral sheet music and explain how each section of the choir must read a certain part. Explain which part has the melody in the song and which parts provide upper and lower harmonies. Go over each part with its section note by note. This will give the choir members an understanding of what each note sounds and looks like. The more practice they have reading music, the more they will begin to identify the specific sound of each written note.
Provide each member of the choir with her starting note and ask her to try to sight read the music a capella from that point. To make each person feel comfortable, explain that this is not an easy task and that many people make frequent mistakes when they are first learning. Explain how to predict the sound of the next note by looking at how many steps it is away from the current note. For example, if you are singing a C and you see that the next note is an E, you know you must go up a third. It is often easier to conceive of music in intervals than to guess at random notes.
Hold sight-reading tests on a regular basis so the choir members can practice and improve their skill. Work with some members individually if they do not progress at the same rate as their peers. Explain that much of sight reading is confidence and that it is better to get a note wrong and learn from it than to shy away from singing it completely. Sight-reading requires confidence, which comes from practicing frequently.