You've been handed a GA4 account someone else set up, or you set it up a year ago and aren't sure how far to trust it. Either way, where do you start?
Opening the reports and hunting for problems won't get you far. GA4 shows hundreds of numbers, and most of them look fine even when the setup is broken. Audit it layer by layer instead, starting with the basics (is the tag firing?) and finishing with the finer points (do your channel groups reflect how you acquire customers?).
Work through the layers in order, because the later layers only mean something once the earlier ones are fixed. There's no point refining channel grouping if internal traffic is still polluting the source data.
Each chapter covers one layer in depth. Read them in sequence, or jump to the layer you need to fix now.