To use the data validation list function in Excel, follow these steps:

1. Select the cells in which you want to create a dropdown list
2. Navigate to the 'Data' tab in the ribbon
3. Click on Data validation from the Data tools section
4. Choose the validation criteria; select list
5. Enter the source of data for the validation list in the source field. This can be an external resources, internal resources, or it can be typed in directly.
6. Select the 'In-cell Dropdown' checkbox
7. Check and customize other parameters as needed
8. Click 'Ok' to complete set the data validation