Maybe you already know what kind of SSE you would like to start (probably a co-op, a Community Supported Agriculture group (CSA), an intentional community,...). You can find specific guidelines on the Internet. If you don´t know yet, such guidelines can nonetheless be helpful and inspirational on your way towards founding a SSE:
5 questions to ask yourselves, if you are planning your co-op:
https://www.uk.coop/developing-co-ops/start-co-operative/plan-your-coop
10 things to consider when you start a CSA (Community Supported Agriculture):
https://communitysupportedagriculture.org.uk/wp-content/uploads/2018/06/S.pdf
7 aspects for developing an intentional community:
https://www.gemeinschaftskompass.de/wp-content/uploads/2018/04/Compass-Short-Beta.pdf