Start with the inventory, which most businesses cannot produce from memory. Pull ninety days of bank and card statements and mark every recurring charge, then search your email for renewals and receipts, since annual subscriptions will not appear in a ninety day window and those are the ones that surprise people. List each with its cost, renewal date, what it does, and who has access. The list itself usually produces the first finding.
Then check ownership on everything, because this is where the expensive problems hide. Your domain, your hosting, your advertising accounts, your analytics property, your business listing, and your social profiles should all be registered to you or to the business rather than to a contractor or a former employee. Discovering that somebody else controls your domain is far better done now than during a dispute, and correcting it while relationships are good is straightforward.
Review access next. Who can reach what, whether anybody retains access they no longer need, and whether anything is still on a shared login. Access accumulates quietly, and a year in there is usually at least one person or one tool with permissions nobody would grant today.
Check that measurement is actually working rather than merely installed. Submit a test through your own contact form and confirm it arrives and registers as a conversion. Verify that analytics is recording and that your own visits are excluded. A meaningful proportion of businesses discover at this point that the form has been broken for weeks or that conversion tracking was never configured, which invalidates a year of marketing conclusions.
Finally, verify the things you are trusting without evidence. Restore a file from your backup rather than confirming the backup exists. Check that your listing hours are correct. Confirm your renewal prices have not increased. Each takes minutes and each is a belief rather than a fact until you have checked it.
Write down what you found and what you changed, because next year you will do this again and the comparison is where the value compounds. A short record naming what was removed, what was corrected, and what you decided to leave means the following audit starts from a position rather than from scratch, and it surfaces the things you decided to tolerate two years running, which is usually a signal to deal with them.