Ah, plumbing. I see it as some sort of a black art.
The game plan is to not make it worse than when you started LOL
Truth be told; I rather enjoy plumbing and have a load of spare parts, pipes and tools. Armed with a little bit of know how I can be devastating.
A trick I have is when I don't know something such as the correct hot water cylinder pressure relief valve and the cold water expansion valve that goes it it, I hang around at the plumbing supply shop and ask someone who looks like a real plumber. Every time they have been very happy to talk plumbing