To enable HDR in your project, you simply need to define which Audio Bus or Busses will act as HDR subsystems.
To enable HDR on an Audio Bus:
Inspect an Audio Bus.
Click the HDR tab of the Property Editor.
Click Enable HDR.
The volume of all sound structures routed to the HDR bus, or to one of its descendants, is affected by the HDR bus. Sounds that are not routed to the HDR bus are not, but can coexist with sounds routed to the HDR bus within your sound design. You can also have more than one HDR bus in a project, as long as they are completely independent: an HDR bus may not have another HDR bus as an ascendant or descendant.
As mentioned previously, sounds at the top of the HDR window will come out at 0 dBFS. Use the bus volume slider of the HDR bus to scale down the output of the HDR system before mixing it with other non-HDR sections of your project.