For security reasons, you may want to have a captcha form on the checkout page. Having this will help incoming order spam, and helps make sure whoever puts in an order is an actual person.
It is a straightforward process, but you will want to determine where you would like the captcha to show up. On the default themes, there is a Top and Bottom widget area. You can feel free to put the recaptcha widget in either of these. If you would like to display it elsewhere than the defaults, you will want to create a separate widget area.
To choose where to show this on the page, go to One Page Checkout > Widgets. Add the reCAPTCHA widget to the respective widget area.
If the widget is put in the default "Bottom" area, the widget will show near the bottom underneath the cart summary.
And that is it! Now any customer will need to insert the characters from the captcha form to checkout. This will aid you and your store from having malicious bots creating fake orders.