Click on the Certificate tab within the menu options for the activity. See Managing Uploaded Content in the Content Manager guide if you are unsure where this is.
Choose the Award certificate on completion radio box and click on Save Changes.
Click on the Preview Certificate button that appears in order to preview it (NB your site settings will dictate how it looks so it may be different to the screenshots in this guide).
If you are happy, you do not need to make any further changes however if you would like to add an additional message, you can do so using the Optional Certificate Message on the Certificate screen and clicking Save Changes.
Preview the Certificate to check your amendments.
The certificate will now be awarded when the course has been completed.