You can add reminders to your Google Calendar using the mobile app or web version of the application. To start, tap the plus sign at the bottom right and select "Reminder" instead of "Event". You can set a date, time, or recurring reminder, and then save it. Reminders also appear in Google Keep, allowing you to add more detail before tracking them using the calendar. Completed reminders are marked as done and appear crossed out on the day they were completed, making it easy to review your accomplishments.