Cut two 5cm x 2cm strips of fluffy side heavy duty velcro tape. These will be placed on the top side of the battery pack – this is the side with 4 small holes on it.
Peel the protective sheet off of each piece and stick them to the top side of the battery pack in parallel. Placing them closer to the outer edges will provide more stability, but be sure not to cover the pack's indicator LEDs – they're quite helpful :)
Line the cable ties up with the adhesive tape on the battery pack and press them securely in place. Ensure the rough/hook side of the cable ties attach to the fluffy loop strips on the top of the battery pack.
Position the battery pack against the Quest's back-bottom strap with the pack's USB ports pointed to the left side.
Bring the long end of the ties around the strap, and pass them each through their respective eyelets. Pull them both tight and press in place to secure.
Connect the USB cable's USB-A connector to the battery pack.
Gather any extra cable slack in a loop and secure it to the Quest's left side strap using another cable tie.
Plug the USB-C end of your cable into the Quest's left side port. You're now ready for your next marathon VR session.
Because the battery pack counters the weight of the head-mounted display, you should be able to loosen the Quest's side straps a bit, without any risk of it falling off. This will reduce the amount of pressure applied to your face and increase comfort – always a good thing :)