WARNING: This page is about a different car, the 2023 BMW X5, 2022 BMW X5, 2021 BMW X5, 2020 BMW X5, and 2019 BMW X5. However, it is still accessible from the selected car via links, so may be relevant.
Place the rear seat and while doing so feed in the lower straps (3).
Connect the connector in the areas (2) and lock it.
Clip the rear seat in the areas (1) into the detents downwards.