Add an event/reminder in the timeline of an opportunity

1. Go into the respective opportunity
2. Click on Timeline as shown in the image:
   
3. Click on button.
4. Fill in the needed details in the form.
   
5. Click Create.
6. If you wish to add this event in your regular calendar. Hit the icon as shown in the image:
   
7. Select your desired calendar from the list .
8. This will pop up your calendar window like below:
   
9.  Hit Save.