Write your letter, choose who can sign it, and publish.
We recommend you to start a google doc with one or two other people. It's a co-creation process! Once you are happy with it, copy paste it here, publish it and share the URL with your collaborators to be the first to sign it.
An open letter is a great way to ask something to your company, organization, institution, city, government. Make sure that it is signed by people that are relevant to what you are asking.
No. That's why we recommend you to carefully share it to the right people at the beginning, so that they will get the opportunity to be first people showing up as the co-signatories.