In order to use Screenful your Trello boards have to exhibit a flow of tasks through at least three states.
Not started - work that has not yet been started
In progress - work that has been started but not completed
Done - work that has been completed
There can naturally be more than three lists on your board. For example you can have states like "in planning", "in review" etc. Screenful is compatible with any workflow such as these example workflows by Trello.
Below is a sample Trello board for development teams
In the above board, there is one list that contain tasks that are not started yet: Incoming bugs. That represents the Not started state.
Once the work is started on a task, the card progresses through In Progress, QA, and Ready for launch. Those represent In progress states.
Once the tasks is completed, the card is moved to the Live list, which represents the Done state.