Go to : https://elegantt.com
Click on "Install the Chrome extension"
Go back to your Trello boards and click on the new Elegantt icon next to the board title.
Log in with your Trello account & approve the app permissions.
Do you want to enable Elegantt on this board ? YES, please !
And that's it!
Every change to Trello is reflected in Elegantt: cards, lists, assignments, etc.
You can choose whether or not to generate an Elegantt schedule for each of your boards.