Reconcile every bank and credit-card account, month by month, until QuickBooks agrees with the statement and the difference is zero. Reconciliation is what turns a pile of entered transactions into books you can trust — the spine of the whole catch-up.
Use the built-in Reconcile tool: enter the statement's ending balance and ending date, then tick off each transaction that cleared, exactly as it appears on the statement. When the difference reaches zero, that month is proven. When it doesn't, the gap is telling you something specific — a missing transaction, a duplicate, a wrong amount, or a payment still in transit at the statement date. Resolve it before moving on; an unreconciled month poisons every month that follows it. Intuit's official walkthrough of the process, the QuickBooks reconciliation help center, covers the mechanics screen by screen. Reconciling as you go, rather than entering everything and reconciling at the end, is what lets you catch an error while you still remember the month it belongs to.