I am a k-12 teacher in Massachusetts and I can give you some guidance in letting you know what you might encounter. All states have their own specific rules but the starting of a Montessori school is largely no different than starting any other private school. You would be looking a pretty big task ahead of you.
Overall, your biggest steps would be finding the market and going through the step to higher the staff and design the curriculum. Of course all of this can be done more easily with enough capital. Largely though, a Montessori school operates as a private school so you would be exempt from a lot of the governance and bureaucracy of local districts.
In California, you must register your school and depending on where your students come from, you may need to be certified from the California Department of Education. The other aspect to consider is hiring teachers. Because you are a private school, your teachers would be exempt from needing a teaching certificate. This would lower the overall cost of hiring teachers but you would be drawing from a less selective pool of candidates. The choice would be up to you. Private schools in California are also exempt from the usual attendance laws so that gives you flexibility in the days students would attend.
There is also items to consider or transportation, medical records, general record keepings, food/nutrition, and technology needs.
Certainly this is not an insurmountable task. Just a lot to consider when you are moving forward. For a more detailed list here is a link that may help: http://amshq.org/School-Resources/Starting-a-School/~/media/623BDCC896DC47B5B8E0D3DC57F834BE.ashx