The QuickBooks Connection Diagnostic Tool is a helpful tool for troubleshooting network-related issues with QuickBooks. To use the tool, first download and install it on your computer. Then, open the tool and select the type of issue you're experiencing, such as network connectivity or multi-user access. Follow the prompts to diagnose and resolve the issue. If you're unsure about how to use the tool or need further assistance, you can contact QuickBooks support for additional guidance.