Create or remove a drop-down list - Excel - Office.com

Create or remove a drop-down list - Excel - Office.com:

Select the cell where you want the drop-down list.
On the Data menu, click Validation, and then click the Settings tab.
In the Allow box, click List.
To specify the location of the list of valid entries, do one of the following:

If the list is in the current worksheet, enter a reference to your list in the Source box.
If the list is on a different worksheet in the same workbook or a different workbook, enter the name that you defined for your list in the Source box.

In both cases, make sure that the reference or name is preceded with an equal sign (=). For example, enter =ValidDepts.

Make sure that the In-cell drop-down check box is selected.
To specify whether the cell can be left blank, select or clear the Ignore blank check box.