Steps
Open Assessment → Back-Log.
Find the unit. Use the search box, or look for the earliest due badges.
Click Start on the card (or in its right-hand panel).
The card shows a green "Started" and slides away. The unit is now in the first column of the Activity Board.
To start the unit and go straight to work on it, use Start & open instead. It opens the unit screen.
You'll need
Permission to start assessments.
A published Assessment workflow. See Workflows.
What next
Work through the unit on the unit screen, then move its card through the board's columns until it's complete.