1. Decide which form you need. A walk-up sale with no prior invoice uses a Sales Receipt. Cash paying down an invoice that's already out there uses Receive Payment. Using the wrong one either skips invoicing entirely when you needed the paper trail, or creates a phantom open invoice when there wasn't one.
2. For a cash sale, open a Sales Receipt. Click + New, then Sales Receipt, pick the customer, and set Payment method to Cash.
3. Add what was sold. Enter each product or service line the same way you would on an invoice, and QuickBooks totals it automatically.
4. Choose where the cash lands. Deposit to Undeposited Funds if it'll be bundled with other cash and deposited later, or point it straight at a specific cash or bank account if it's going in on its own.
5. For cash against an existing invoice, use Receive Payment instead. Pick the customer, check off the invoice being paid, set the payment method to Cash, and enter the amount actually received.
6. Save, and keep a physical record if you need one. QuickBooks itself doesn't require a signature or receipt slip, but if your business hands out a paper receipt for cash, that's a step outside QuickBooks you'll still need to handle separately.