Excel itself doesn't have a built-in feature for "input masks" like some other software. However, you can achieve similar functionality using Data Validation with custom formulas. Here's how:

1. Select the cell(s): Choose the cell(s) where you want to restrict data entry based on a specific format.

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

3. Settings Tab: In the "Settings" tab of the Data Validation window, choose "Custom" under the "Allow" dropdown menu.

4. Custom Formula: This is where you define the rule for allowed data format. Here's the basic structure:

```excel
=AND( <condition1>, <condition2>, ... )
```

- Replace `<condition1>`, `<condition2>` etc. with specific formulas that check if the entered data matches your desired format.

5. Common Input Mask Scenarios and Formulas:

- Limit characters: Use `LEN(A1) = X` (replace A1 with your cell reference and X with the allowed number of characters).

- Allow only numbers: Use `ISNUMBER(VALUE(A1))` (ensures numeric input).

- Specific format (e.g., XXX-XXX-XXXX): Combine multiple conditions using `AND` and functions like `LEFT`, `RIGHT`, `MID` to check characters at specific positions.

6. Input Message (Optional): In the "Input Message" tab, you can provide a clear message that will be displayed when users hover over the cell, explaining the expected data format.

7. Error Alert (Optional): In the "Error Alert" tab, you can customize the message displayed if users enter invalid data.

8. Click OK: Once you've defined your formula and any optional messages, click "OK" to activate Data Validation.

Remember: Building the custom formula will require knowledge of Excel functions like `LEFT`, `RIGHT`, `ISNUMBER`, and logical operators like `AND`, `OR`. You can search online for specific formulas based on your desired input mask format.