How to Integrate Slack With Trello

Step 1: Install the Trello App on Your Device, You first need to have to download the Trello app from the Slack App Directory.

Step 2: After this, you’ll again see the Add to Slack option on the app page on the displayed Trello website.
Follow this up by choosing a Trello team to link to Slack. After this, click on Add to Slack on the right-hand side of your team.
Once that’s done, select Allow to grant Trello access to your Slack.

Step 3: Link Your Slack Account to Your Trello Account, by Click on Apps located in the left sidebar menu on your desktop version of Slack.
If you don’t see this option, you’ll see the three horizontal lines or More options to find it.

This will bring up the Apps page in your Slack window. Search for “Trello” using the search bar, and then select it. This will open a direct message with the app.

Step 4: Once that’s done, click on the Lightning bolt icon located to the left of the message field. You’ll now see the Shortcuts menu.

Search for and select /trello from the menu to add the slash command to the message field. Send the message after that.
And finally… select Link Trello account. Then click on Allow, followed by Link Accounts.

Credit : https://nira.com/how-to-integrate-slack-with-trello/