1. Press & hold shift and select which items you want to move to a sub-board. These selected items will highlight in green.
NB. If you want to select all the items in the board, press SHIFT and click ‘Select All’ once the bulk actions drawer box pops up at the bottom of the screen
2. Click ‘Add to sub-board’ bottom right corner
3. Give the board a name
4. Hit ‘Create Board’
5. Hit ‘Cancel’ bottom left to exit the bulk actions drawer
Your new sub-board will be at the top of the main board :)