Couple things to note about having a sellers permit:
You don't need to be a LLC or anything to have a sellers permit. If you're just one guy, you're a sole proprietor, and don't need to do anything special to still just be a business.
It's super important to keep track of your sales by location. This means recording the amount you made in different cities such as Oakland, Berkeley, SF, etc. When it's time to file your sales taxes, you'll need to categorize them by city and county where the sales occurred. I have a spreadsheet listing the events we participate in and how much we made at each one, along with the corresponding city and county. This way I can easily split sales by city and county during tax time.
You get your sales permit by making an account with CDTFA, (the california department of tax and fee administration)
Ok onto how to get a sellers permit:
uhhh so I didn't make this walkthru but it's the best one out there. I'm not 100% sure who made it but I'll source them as soon as I figure it out.