In Excel, data validation allows you to set up informative messages to guide users without completely stopping data entry. Here's how to achieve a warning message:

1. Select the cells: Choose the cells where you want the warning message to appear.

2. Data Validation: Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.

3. Input Message Tab: Click on the "Input Message" tab within the Data Validation window.

4. Show message: Check the box next to "Show input message when cell is selected."

5. Craft your message:
- In the "Title" box, enter a brief and clear title for your warning message (e.g., "Warning: Double-check data format").
- In the "Input message" box, type the specific warning you want users to see when they select the cell (e.g., "Ensure you enter data in YYYY-MM-DD format").

6. Error Alert Tab (Optional):
- While not strictly necessary for a warning, you can also set up an "Error Alert" message in the next tab. Here, you can choose "Warning" as the style, allowing users to enter invalid data but prompting them with your message before accepting it.

7. Click OK: Click "OK" to close the Data Validation window.

Now, whenever someone selects the cell, they will see your warning message in a small popup window. They can still enter any data, but the message will serve as a reminder for proper input.

Note: This approach relies on user awareness and doesn't enforce data format. Consider combining it with conditional formatting to highlight incorrect entries visually or using data validation with custom formulas for stricter control if needed.