Great question — answering as the UphillTech team:
Insert the taps in the hot and cold lines going to the shower. Given your level of experience, I would suggest you use only prefabricated flexible lines made with connectors that match the existing ones. Most problems arise from leaks due to imperfect connections. As usual, follow the instructions, especially with regard to tightening (neither too much nor too little). You may benefit from YouTube or other online guidance.
Full answer by MadMonty: https://diy.stackexchange.com/a/301903 (CC BY-SA 4.0).
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