The standard is published by British Standards Institute, covering reporting of Scope 1 and 2, and if possible, Scope 3, emissions by an organisation. With care, a little time and some knowledge it is practical to achieve for any size business.
Assuming you have followed the correct procedures, you simply need to know the amount and source of electricity used by your business, as well any fuel used directly, and have an insight into emissions in your supply chains. Complete knowledge of all emissions is not required, but clear transparency is essential. Statements of fossil fuel emissions can be verified by external organisations like Intsilo. Such certifications can then be used to help promote your business to your customers.