You can add subcolumns only on a paid plan with a connected Scrum or Kanban module.
Click the plus sign to the right of the column name. Select "Add 2 subcolumns"
Column will split into two (Doing and Done) and all cards from parent column will be moved to first subcolumn.
For more subcolumns search for “Add subcolumn” menu item in column's context menu.
If you need to return one level, you can merge subcolumns from parent column's context menu. Click on the plus sign to the right of the column name → Manage subcolumns → Merge subcolumns.
All cards in subcolumns will be gathered in parent's one.