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
From the name drop down menu, Create New
OR
Select the Business Unit Tree to copy from
Then click the drop down again, from the cog next to the name select Copy,
This will create a [Copy Original Name] Tree
Click the drop down again, click the cog and choose Rename Tree
Give your tree a new name by overtyping, SAVE CHANGES
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