If you do not yet know whether you have to report: the deadline checker answers it in about a minute and does not require an email to begin.
If you know you must report and have never done it: read the carbon accounting guide, then Scope 1, 2 and 3 explained. Between them they cover what is being measured and why it works differently here.
If you know the measurement and need the requirement: the compliance hub explains the rules and the ESG report checklist tells you what to have in hand before drafting.
If your first mandatory year is close and you have not started: go straight to preparing for the 2028 deadline. It works backwards from the filing date and is deliberately blunt about what is not recoverable.