Jason,
The function of excel you're wanting to use is called 'data validation'. I use excel 2003 so expect you're using a newer version I'm not familiar with so I can't give you the step by step.
1. In two spare cells insert the characters you want to appear in the dropdown (a tick and an x in this case)
2. Select the range of cells where you want the dropdown to appear
3. Apply data validation rules to these cells (do a search for 'data validation') to find out the exact buttons to press on the ribbon. Alternatively use the following keyboard shortcut (Press Ctrl then D then L).
4. The type of validation you want is 'list'
5. When you choose list, you will be prompted to select the cells where the characters you want in the dropdown are located (from step 1).
6. You should be good to go