I’m not going to step by step this, but a battery replacement is about the easiest thing you can do to an iPhone. There’s some fiddly bits with small screws and ribbon cables, but it’s not actually difficult. I’ve also done screens, speaker assemblies, and microphone assemblies which are all much the same level. The only thing which will properly kill an iPhone in an irreparable way is it to drown the main circuit board, and with recent models being waterproof to some extent even that isn’t so much of a problem.