Flat has a sophisticated built-in automated importer to pull in data from other tools. To import data into Flat, click your avatar in the upper right corner and visit Settings, then visit the Admin Settings | Import Data section.
The Admin Settings section is only visible to team members with the Admin role.
You'll be redirected to Trello and asked to grant Flat temporary read access to your Trello account. If you approve, you'll be redirected back to Flat where you can select which Trello board to import.
To import multiple Trello boards, import each board one at a time by clicking the Import from Trello button.
For very large Trello boards with thousands of cards, the import may take take a few minutes.
The import includes Trello card titles, descriptions, members, labels, checklists, images, attachments, and comments.
The import includes Jira issue titles, descriptions, assignees, watchers, labels, attachments, and comments.