You can learn how to enable integrations here.
Once enabled, you must then configure the integration on each board or backlog that you want to post notifications into Slack from. From the board or backlog hamburger menu, choose the App Store and Integrations.
After clicking Configure you will be prompted to log into your Slack account to choose the channel in your Slack team to post to. After finalizing the configuration in Slack, the initial setup is complete and Favro will begin to post updates to the chosen channel.
It is also possible to choose which notifications are posted to Slack. You can limit notifications to one column on a board or specific events.