How do I add a data validation rule to a Google Sheets spreadsheet?

Data validation rules are an important tool for ensuring that data entered into a spreadsheet is accurate and consistent. By adding a data validation rule to a Google Sheets spreadsheet, you can ensure that the data entered into the spreadsheet meets certain criteria, such as being within a certain range, being a certain type of data, or being a certain length. This can help to reduce errors and ensure that the data is accurate and reliable.