You've been handed a GA4 account that someone else set up. Or you set it up yourself a year ago and you're not sure how much to trust what it's showing you. Either way, the question is the same: where do you even start?
The answer is not to open the reports and start looking for problems. GA4 can show you hundreds of numbers, most of which will look fine even when the underlying setup is broken. The right approach is a structured audit, working through the account layer by layer, from the most fundamental (is the tag firing?) to the most nuanced (are your channel groups configured to reflect how you actually acquire customers?).
This guide gives you that structure. Work through it in order. The fixes in the earlier layers make the later layers meaningful. There's no point refining your channel grouping if internal traffic is still polluting the source data.
This is a multi-chapter guide. Each chapter covers one layer of the audit in depth. Read them in sequence for the full picture, or jump to the chapter covering the layer you need to fix right now.