Log in with your net-ID.
Put your mouse cursor over the calendar date you want to post in. A little "Add a new event" button will appear. Click on it. On your right-side menu bar, the box "Select Calendar" will appear.
Click on the drop-down menu, and choose the exact calendar you want to post on (academic, social, etc.). Click next.
Fill in the event details.
Click add, and you're done!