In the store admin click Store Settings → Checkout → "Place Order" section
Add something like this in the text editor:
If you like you can add a little flare by including a check box icon before the text. Do this by opening the View → Source Code menu and put this in before your text:
<em class="fa fa-check-square"> </em>
Click Save and check the last step in your store called "Review & Confirm" to see your change.