1. First, we open excel on the computer, and then we randomly select a cell.

2. Then we click Insert in the toolbar;

3. The interface pops up, and we click the symbol;

4. In the pop-up interface, we click the drop-down arrow next to the subset;

5. The interface pops up, and we click the geometric symbol;

6. Then we click on the small blank box, and then we click Insert;

7. The result is as shown in the figure, so we inserted a small blank box, and then we can click Close.