You can register here and send in your “position letter” on any bill in the Legislature. The system forwards your message to each member of each Committee that is going to hear the bill.
https://calegislation.lc.ca.gov/Advocates/
Your position letter can be a 5 page pdf file. There’s no page or word limit but as when communicating with anyone brevity is a virtue. I put the main point of my message in the first paragraph and a summary of the letter in the first or second.
I have heard that the Committees treat position letters from organizations differently from position letters from individuals. That is, they list the organizations supporting or opposing a bill but not the individuals.
What’s a letterhead? Something you make up on your computer, same as the organization and its name. You could be the Executive Director of the [Name of your street] Residents for Safe Technology and not even know it! Well aren’t you glad you discovered it?
Your letterhead can consist of the organization’s name, a website, and a mailing address (which can be a post office box). Doesn’t even need to have a mailing address.