The power BFF stacks on top of the QT Py using the included female headers.

The data pin from the LEDs is connected to the yellow stemma wire (SCL) and plugs into the QT Py via the Stemma port.

The LED strand and the power supply's power and ground wires both get connected to the Power BFF.

In this diagram the LEDs are wired into the screw terminals and the power supply is plugged into the 2.1mm jack. As long as both are connected, these two ports are interchangeable, so use whichever makes sense with your power supply.