Select a Context that is sorted by rank (e.g. look for a context named 'Rank'). If one does not exist, you can either create a new context or modify the 'On The Fly' context to sort by the 'Rank' field.
Your issues will be ordered by rank. Note that sub-tasks can only be ranked within the parent task.
You can now rank your issues by dragging and dropping the cards on your Planning Board as desired see below.