Here's how to create a custom error message for data validation rules in Excel:

1. Select the cell(s): Choose the cell(s) where you want to implement the data validation rule and the custom error message.
2. Data Validation: Navigate to the "Data" tab on the Excel ribbon. In the "Data Tools" group, click "Data Validation."
3. Settings Tab: This is where you define the validation criteria. Choose the appropriate validation rule based on your needs (e.g., Whole Number, Text Length, Custom formula).
4. Error Alert Tab: Click on the "Error Alert" tab. This is where you can customize the error message users will see if they enter invalid data.
5. Title: In the "Title" box, enter a clear and concise title for your error message. This will help users understand the nature of the error. (e.g., "Invalid Input")
6. Error message: In the "Error message" box, type your custom message. Be informative and explain what kind of data is expected in the cell. (e.g., "Please enter a number between 1 and 100.")
7. Style (Optional): You can choose the style of the error message popup window from a dropdown menu (e.g., Stop, Warning, Information).
8. Show Input Message (Optional): If you want to display a helpful message when users select the cell (explaining what kind of data to enter), check the box next to "Show input message when cell is selected" in the "Input Message" tab.

Example:

Let's say you want to ensure a cell only accepts ages between 18 and 65. Here's what your settings might look like:

- Settings Tab: Choose "Between" and define the minimum and maximum values (18 and 65).
- Error Alert Tab:
- Title: "Invalid Age"
- Error message: "Please enter a valid age between 18 and 65."

Tips:

* Keep your error message clear, concise, and easy to understand.
* Use friendly language to guide users towards the correct data format.
* You can use cell references within your error message to dynamically display specific requirements based on other data in your sheet.

By creating informative custom error messages, you can improve the data quality in your Excel spreadsheet and make it easier for users to enter valid information.