How do I go about fitting another job in my work schedule that i now have?

I have a job as being a assistant manager, and I had another store call me for a posistion as a sales associate, but my hours were I already work are flexible. How can I work another job? My job now is first, so if I got another job how can I fit it inm my work schedule now with my hours not being set stable?|||Yes, try to have your set shift as early as possible.|||Set one as a priority, you assistant manager job. Let the part time job know this. Then, as soon as you get your schedule for you primary job, give it to the part time job to schedule around.