As an admin you will be able to access your internship management capabilities from the left hand menu of your group dashboard. To create an internship, click on Internships > Add Internship.
From here, you will begin the internship workflow where you can add basic information such as the description, internship period, etc.
After completing the basics, you can decide details such as the privacy level, whether or not it should applicants will be paid, and if the internship counts for course credit, etc.
In order to publish the internship, be sure to add your "How to Apply" details and application questions.
Finally, once your required settings are filled out, you can publish for your applicants to begin applying.