Enable the Developer tab:
Go to File.
Click on Options.
In the Word Options window, select Customize Ribbon on the left pane.
Under Main Tabs, check the box next to Developer.
Click OK.
Insert the checkbox:
Place your cursor where you want to insert the checkbox.
Go to the Developer tab.
In the Controls group, click on the Check Box Content Control button.
A checkbox will appear at your cursor location. You can click on it to mark it with an "X".
Additional options:
You can copy and paste the checkbox to other parts of your document.
To change the default "X" to another symbol, right-click the checkbox and select Properties. In the Content Control Properties window, click the Change button next to Checked symbol and choose a different symbol.
You can format the checkbox by selecting it and using the formatting options on the Home tab.