The fixings go through the flat part of the trough, there are a couple of length options for going into wood linked below.
https://www.roofingsuperstore.co.uk/product/cladco-32mm-screw-to-wood-19mm-bonded-washer-pack-of-100.html
https://www.roofingsuperstore.co.uk/product/cladco-45mm-screw-to-wood-19mm-bonded-washer-pack-of-100.html