Figure out what sort of music your group will perform.
Create flyers and advertisements to attract members. You can hold auditions at a designated time, or have the prospective members schedule their own times. An online classifieds site can also be a great place to advertise. Talented friends can also make a great addition to a singing group.
Write songs, have a friend write songs, or decide on songs that your group might like to cover.
Practice and rehearse to perfect rhythm and intonation. Continue to practice until your group has formed a cohesive sound.
Arrange performances. You can try calling local coffee houses and bars, or arranging joint performances with other bands that you know.