Making a list of the items you want to appear in the drop-down, but anyplace in your spreadsheet, is the first step. For example:
![enter image description here](https://i.stack.imgur.com/Nofms.png)
Then the next step is to click the "Data Validation" button in the ribbon :
![enter image description here](https://i.stack.imgur.com/uf3yN.png)
Ok, step 3 is to click "Settings" in the "Data Valid" menu, and choose "List" from the drop-down:
![enter image description here](https://i.stack.imgur.com/Ynj6k.png)
Step 4 is to click on that little blue icon in the "List" sub-menu :
![enter image description here](https://i.stack.imgur.com/2fCxj.png)
The next step is to simply highlight the Cells you need. ![enter image description here](https://i.stack.imgur.com/cZ70T.png)
Finished now. Here is the dropdown :
![enter image description here](https://i.stack.imgur.com/mBgFt.png)