The ultra-short answer is: make a proposal on how to run sociocratically and make the decision to be sociocratic by consensus - because that’s your current decision-making system.
What comes along with that is also to get work for acceptance of the new system, write and get input on a new structure, and train everyone.
You will need a governance agreement that lays out how you describe sociocracy in your particular organization. (We have samples!)
For more information, check out our handbook Many Voices One Song chapter 6 or our page on implementation.