Here's how to display a custom warning message before data entry using data validation in Excel:

1. Select the cell(s): Choose the cell(s) where you want to prompt the user with a warning message before entering data. You can select a single cell or a range of cells.

2. Data Validation: Go to the "Data" tab on the Excel ribbon. In the "Data Tools" group, click "Data Validation."

3. Settings Tab - Allow: In the "Settings" tab of the Data Validation window, you won't be setting restrictions on the type of data allowed here. Leave the "Allow" section blank or choose "Any value" depending on your preference.

4. Input Message Tab: Now, switch to the "Input Message" tab.

5. Show Input Message: Check the box next to "Show input message when cell is selected." This will display your message whenever someone clicks on the cell.

6. Title (Optional): In the "Title" box, you can enter a brief and clear title for your warning message (e.g., "Data Entry Warning"). This will appear in bold at the top of the message window.

7. Input Message: In the larger "Input message" box, type your custom warning message. Here, you can explain the expected data format, potential errors, or any other relevant information to guide users before entering data.

8. Click OK: Once you've entered your title and message, click "OK" to close the Data Validation window.

Now, whenever someone selects the cell(s) with the data validation rule, your custom warning message will appear as a small pop-up window. This helps users understand the specific requirements for entering data in those cells and reduces the risk of errors.