Log in with your members-only username and password.
Click "Final Examination" on the left-hand side of the page (it's slightly larger, located above "Questions/Comments").
If you haven't already passed the exam, take and pass it.
Once you've passed the exam, a link to "Download your certificate" will show up on the "Final Examination" page (you may need to refresh the page after passing the exam). You can also go here: http://www.nachi.org/my-exam-sessions.htm .
You can download and print your certificate whenever you want.
The image is a high-resolution JPEG image (300 pixels per inch), so it might take a while to download. Most browsers should prompt you automatically to download the file. If not, you can right-click the "Download your certificate" link and choose "Save Target As..." or "Save Link As..." to download it.