1. Export the report you already have
Take a campaign report, and a search-term report if you have one, out of your advertising account as a CSV for a defined period. Include spend, impressions, clicks, conversions and conversion value where your account records them.
2. Check the data before you read it
Confirm one currency, one reporting grain and no duplicated rows. An audit built on mixed currencies or part-exported columns will read as a performance problem when it is really an export problem.
3. Reconcile the totals
Add the rows up and compare them with the account totals for the same period. Where a figure was never exported, record it as unavailable — not as zero. A zero you invented becomes a decision you regret.
4. Look for the patterns that cost money
Spend concentrated in one campaign, spend with no recorded conversions, cost per conversion drifting against the first half of the period, return on ad spend under your own target, and search terms that spent without converting.
5. Write down the method and the limits
Every finding needs the calculation behind it and what it cannot tell you. A finding with no method is an opinion.
6. Decide, then act in your own account
Approve, defer or decline each next step, make the change yourself in your advertising account, then re-import the following period and measure whether it moved.