This video from Crosswater demonstrates the basics of how to install an SC50-T20 thermostatic cartridge into a Crosswater Dual control mixer valve. It also demonstrates the fitting of the flow control cartridge.
The SC50-T20 cartridge is also used in shower valves from shower brands such as Ultra and Hudson Reed. Whilst the fitting process will be similar, some of the steps and parts will differ slightly between brands. In this case, please use this video as a rough guide to understanding the basic steps.