Start by determining whether you will play in the band and what instrument you will contribute. Make a list of how big of a band you want or how many pieces you want in your jazz band. Indicate which instruments are missing.
Run through the musicians you know in your mind and who you could possibly ask to play in a jazz band. Think of individuals who have some knowledge of jazz music, typically university students and faculty are interested in playing in extracurricular bands.
Post a flyer indicating auditions for a forming jazz band at local schools to gather otherwise untapped talent. Set up a time and place to hold very informal auditions where you will simply play with them to get a feel with how well you will work together in a band environment and their relative level of skill.
Put a notice on websites like Craigslist.com and indicate a need for jazz musicians in your area if the flyers are not yielding results. Choose the individuals for your band and set at least weekly if not twice weekly rehearsals to begin with to establish a rapport.
Practice with your band to develop a repertoire of songs that showcases the talents of your performers. Then choose a good a name for your band. Set up your first gig at a local book store or at an amateur jazz showcase. These venues are typically relatively easy to secure if you are somewhat rehearsed and professional.