Here's how to create an input message for data validation in Excel:

1. Select the cell(s): Choose the cell(s) where you want to restrict data entry and display the input message.

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

3. Input Message Tab: Within the Data Validation window, switch to the "Input Message" tab.

4. Show Input Message: Ensure the checkbox next to "Show input message when cell is selected" is ticked.

5. Title (Optional): In the "Title" box, you can enter a brief, descriptive title for your message (e.g., "Valid Values"). This title will appear in bold within the popup message.

6. Input Message: In the larger "Input message" box, type the clear and informative message you want users to see when they select the cell. Explain what kind of data is valid for this cell (e.g., "Enter a number between 1 and 100").

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

Now, whenever someone selects the cell, they'll see your custom input message explaining the valid data options. This helps guide users and ensures they enter the correct data type for your spreadsheet.