I'm using the one for a BMW E90. Note that it's not officially approved by Schroth, so use at your own risk.
To remove the headrest, follow these instructions:
To remove the rear seat bottom cushion, follow these instructions:
Additionally, here are some pictures of the install and how I mounted it.
For the front, you need to remove this bolt (underside front of the seat, door-side) - then the side panel can pop off:
View attachment 447972
Then you can mount the clip here:
View attachment 447974
Rear mount works on the seat belt bolt as normal:
View attachment 447979
Feed the belts through based on the video trick for the headrest:
View attachment 447977
Done!
View attachment 447973