Use the drums to begin. Place condenser microphones on the snare and bass drums, as well as two overhead condenser microphones for the cymbals and toms. Have the drummer play each piece on his drum kit, while you raise the volume of each microphone on your mixer until they are all at loud enough levels to be heard throughout the room. Have the drummer play around on his kit so you can make sure it sounds balanced.
Move to the guitar. Place a condenser microphone in front of each guitar and bass guitar amp. Have the guitarist play their instruments while you use the mixer to adjust the volume. The bass player will most likely not need too much help from the microphone, as bass frequencies travel far.
Instruct the singer and backup singers of the band to step up to their dynamic microphones on the stage. Have each person sing at their normal volume while you raise the volume for these microphones until they fill the room with sound. It is okay to make the volume a little loud, these frequencies will be absorbed once people fill the venue.
Make sure your positioning is correct. Place the drums in the center back of the stage. Guitar players should be placed on either side. The lead vocalist should be in the center of the stage with backup vocalists to the side. Use the pans above each microphone input on your mixer to replicate this overall positioning by slightly shifting sounds left or right.
Set up the stage lights. If you do not have overhead lights to use as spotlights, then put a few around the outside of the stage. This will at least light up the band when all the lights in the venue are off.
Have the band play all together if there is time. This will give you an opportunity to balance all of the different instruments correctly before the show starts. This way, as each new band comes on stage, you will only have to adjust the master volume, not the balance between instruments.