Here's how to validate data based on a formula in Excel:

1. Select the cells: Choose the cell(s) where you want to restrict data entry using a formula. You can select a single cell or a range of cells.

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.

4. Allow: Under "Allow," choose "Custom" from the dropdown menu. This option allows you to create your own validation rule based on a formula.

5. Formula: In the "Formula" box, enter the formula that will determine whether the entered data is valid.

- The formula should evaluate to TRUE if the data is valid and FALSE if it's invalid.
- You can use cell references, functions, and operators within your formula.

6. Example Formula: Let's say you want to allow only values between 10 and 20 in a cell. You can enter the following formula: `=A1>=10 AND A1<=20` (assuming your data is in cell A1). This formula checks if the value in A1 is greater than or equal to 10 AND less than or equal to 20.

7. Error Message (Optional): In the "Error message" box, you can type a custom message that will be displayed if the user enters invalid data. This helps users understand why their input was rejected.

8. Title (Optional): In the "Input message" tab, you can create a title and an input message that will appear when users select the cell. This can provide guidance on what kind of data is expected.

9. Click OK: Once you've defined your formula and any optional messages, click "OK" to activate the data validation rule.

Now, whenever someone tries to enter data into the selected cell(s), Excel will evaluate the formula. If the formula returns TRUE, the data is accepted. If it returns FALSE, Excel will display the error message you defined, preventing invalid data entry.