Choose your microphones and setup. This is perhaps the most important part of the recording process --- while hopefully you'll have time to experiment a little on the day of recording, having an idea of what technique you're going to use ahead of time is invaluable. Research various microphone setups and test them.
Try a coincident stereo microphone technique. The most popular setup is the "XY" method. Place two cardioid mics on the same stand, one above the other, and angle them 90 degrees apart. Position them in front of the choir with the 45-degree mark pointing directly at the choir's midpoint. This provides a crisp stereo image and good localization, though the sound stage may seem narrow.
Try a spaced-pair stereo technique. The most common is called the "AB" setup. Position two omni microphones side by side. How far apart you position them is up to you, though the wider the gap the more diffused the stereo image and the more indistinct the localization. If you want the two microphones at the extreme corners of the choir, add a third microphone in the middle to prevent an exaggerated sound stage.
Experiment with a part-by-part method, with one microphone for each part of the choir. This requires the choir to use a different standing position than usual, and while this might not sound as realistic as a true stereo recording, it gives you much more control in post-production mixing. A more intimate sound is also possible, produced by having the microphones closer to the individual singers.
Set up your final recording apparatus in the location you intend to use as a studio. Unless you're using a part-by-part technique, the choir should stand on risers. For a stereo technique, position the microphones approximately 12 feet in front of the choir. Raise them above the height of the singers, angled downward to point at the middle row. Plug in the microphones and your headphones to the soundboard. Adjust the sliders so that the left microphone is recording entirely on the left channel and the right on the right.
Have the choir sing a short number and listen to the performance on the headphones. Since you don't have enough microphones to control every individual voice level in post-production, adjust the singers for the best possible sound, moving those who blend well next to each other, and positioning overpowering parts nearer to the back. Soloists should stand in the front. Proceed with your session, making adjustments and rerecording as necessary.