Find other homeschoolers in the area who might want to join your homeschool school. You can do this verbally, with fliers, or through newspaper ads.
Locate a place for you to gather. If your group is small it could be in someone's home, if it's larger, you might need to rent a space. Churches often have rooms they let people use during the week.
Hold a meeting before you officially start to discuss basics with the other parents, including how often you plan to meet and on what days. Create a calendar and small budget to cover your basic costs like building rental. Schedule regular meetings for parents so you can discuss likes, dislikes and rules as your homeschool school evolves.
Peruse your state mandates and discuss curriculum elements you want to cover. If you want to hold first aid courses or do the Presidential Fitness course, find someone qualified to do this or designate a parent to call around to find someone who will come in and teach.
Emphasize that the homeschool school benefits everyone so everyone needs to pitch in to help it run. Make a list of parents and their talents then set up a teaching schedule accordingly.
List the children, their ages and grades then divide them into classes. You could divide by age, grade, or choose equally balanced multi-age groups depending on the desires of the families.
Designate an area for sign-ins and news as the kids come in so you know who is around in case of an emergency. If you are in a building, have at least one adult patrolling hallways to get supplies for teachers, take care of sick kids, or walk someone to a bathroom.