These huge L-shaped brackets are supposed to be mounted to the back of the fascia. At least that was the case with the unit in my X-type. If you haven 't received any screws or bolts you will have to find some that fit. Or buy a fascia that has them included.
Or something like this may help:
https://www.ebay.co.uk/itm/Car-Stere...N/183126284091
You place the frame in the fascia and place it in your center console. After that you place the unit in the frame. It is highly unlikely to get a good fit if you place the radio first and the fascia later. You simply don't have a reliable construction to mount the radio to if you place it first. The fascia and cage will provide a stable fitting.
Pictures of the cage I used for my X-type:
IMG 0008:
IMG 0009:
