A messy QuickBooks file is fixable. The trick is doing the steps in the right order so you are not correcting the same transactions twice.
Start with the opening balances
Pick a clean starting point, usually the last date the file was reconciled or the beginning of the tax year. Everything before it gets locked, everything after it gets fixed.
Reconnect and reconcile every account
Connect each bank and credit card feed, import any missing history, then reconcile month by month from the starting point forward.
- Match feed transactions to existing entries instead of adding duplicates
- Clear the Undeposited Funds account
- Confirm each ending balance against the statement
Fix the chart of accounts
Merge duplicate accounts, remove accounts nobody uses, and make sure categories map to the lines on your tax return. A shorter chart of accounts is almost always a better one.
Clear Uncategorized Income and Expense
Work through the pile with your receipts and a list of your recurring vendors. Set up bank rules so the same transactions categorize themselves next month.
Clean up receivables and payables
Write off invoices that will never be collected, apply unapplied payments, and remove bills that were paid outside the system.
Lock the period
Once the numbers tie out, set a closing date with a password so prior periods cannot be changed by accident. This is the step most people skip, and it is why the file gets messy again.