With the Miro Power-Up, you can easily attach Miro boards to Trello cards, view/comment/edit the boards and create new ones right from within Trello. No need to switch between the tools!
How to install
To enable the Power-Up, open Menu on the right-hand side of a Trello board. Select Power-Ups.
Trello Power-Ups
Find Miro on the list and click Add. 
Installing the Miro Power-Up
How to use the Power-Up
Attaching boards to Trello cards
Open a card and click Miro in the Power-Ups section.
Miro Power-Up
You will see a picker with Miro boards and will be able to switch between your teams on the picker. If you're not authorized in Miro, you will need to sign in first.
Select a board and set the sharing settings for the board in the drop-down menu. You can make the board available for viewing and commenting by anyone on Trello's side.
✏️ For Enterprise Plan Miro users, your access settings will follow organization-wide access controls which might imply that some sharing options may be restricted. Learn more: Managing Enterprise sharing policy for embed integrations.
Setting up access rights for a Miro board
Your board is now attached to the chosen card:
Trello card with a Miro board attached
Creating new boards from Trello
To create a new board from within Trello, click Miro in Power-ups section and create a New board from the picker.
Creating a board from the picker
Viewing, commenting and editing boards
Click the Preview button to view/comment/edit a board depending on the set access rights. The board window will open as an overlay allowing you to work and collaborate as if you were in Miro.
Preview and edit a Miro board in Trello
You can also open the board in Miro in a new tab for your convenience.
Detaching boards
To detach a board, click Remove in the upper-right corner, and the attachment will be instantly removed from the card (on the Miro side the board will be unaffected).
The option to remove the attached board
How to disable the Power-Up
To disable the Miro Power-Up, find Miro among your Power-Ups and click Settings > Disable.
The option to Uninstall the Miro Power-Up in Trello
⚠️ Note that the option to import Trello cards to Miro is not supported.