Select the cell or range where you want the drop-down list
Go to the Ribbon and click the Data tab
Click Data Validation
In the Allow dropdown, select List
In the Source box, do one of the following:
Type items separated by commas (e.g., `Red,Green,Blue`)
Select a range that contains the list items (e.g., `=$A$1:$A$10`)
(Optional) Check In-cell dropdown if you want the arrow visible
(Optional) Check Ignore blank if you want to allow blank entries
Click OK
Test the drop-down by clicking the arrow in the selected cell(s)