To add a drop-down list in Excel, follow these steps:

1. Prepare your list of items you want in the drop-down.
2. Select the cells where you want the drop-down list to appear.
3. Go to the Data tab on the Ribbon, then click on Data Validation.
4. In the Data Validation dialog box, under the Settings tab, choose "List" from the Allow drop-down menu.
5. In the Source box, either select the range of cells that contains your list items or type the list items directly separated by commas.
6. Click OK.

Now the selected cells will have a drop-down list that allows you to choose from the predefined options. You can also edit the list later by updating the source range or the typed list in Data Validation settings.