Here is a very helpful article from DuoLingo:

I hope to start filming the recording of the Greek class in October.  Meanwhile, join DuoLingo, jump into the Greek class and start practicing pronunciation. You will need to know how to type in Greek. Follow the instructions given by DuoLingo below:

HOW TO ADD THE GREEK ALPHABET TO YOUR KEYBOARD Here are some links to easily convert your keyboard to enable you to use both Latin and Greek characters as you need. We have found these easy to install and simple to use. If you have any other links please share them with the community.

For Windows

https://support.microsoft.com/en-us/help/17424/windows-change-keyboard-layout

https://www.conversationexchange.com/resources/keyboard-language.php?lg=en

For Mac

http://www.wikihow.com/Change-the-Keyboard-Language-of-a-Mac

AND the Forums: >https://www.duolingo.com/topic/936