You can either create a new Business Unit Tree, or copy an existing one, that has a Hierarchy of Headings already set up, as well as some of the Business Units already allocated.
Navigate to Reports > Report Tools > Business Unit Trees
ACTIONS at the top right > Create New
OR Copy Tree
This will create a [Copy Original Name] Tree
Click ACTIONS again, and choose Rename Tree
Give your tree a new name by overtyping, RENAME
Make any changes required to the Hierarchy and Headings in the Tree
Allocate the Business Units for all the organisations to the relevant headings
For full details refer to Business Unit Trees