Part 1
Ten minutes in the search terms report
It lists the actual searches that triggered your ads, with the cost next to each. Sort by cost, read the top fifty rows, and mark anything that was never going to buy from you: jobs, free, DIY, competitor research, a country you do not ship to. Most people find something in the first minute, and the money saved is immediate rather than theoretical.
Do this: Open the report, sort by cost descending, and read fifty rows. Do it before you read the rest of this page.
Part 2
Exclude the intent, not just the word
One negative keyword removes one phrasing. A short list of intent words removes a category: free, cheap, jobs, salary, download, torrent, review, how to make. Those cover most of the waste in most accounts, and they keep working as new phrasings appear.
Do this: Build one shared negative list with intent words and attach it to every campaign, not to one.
Halfway, and free
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We read your search terms report and send back the list of what to exclude, in the order that saves the most. One email, no call.
Part 3
Match types matter more here than anywhere else
A broad match negative blocks more than you expect and an exact match negative blocks almost nothing. This is where careless exclusions do damage: someone blocks the word "cheap" as broad and quietly kills a set of searches that were converting.
Do this: Add negatives as phrase match by default. Reserve exact for terms you are certain about.
Part 4
The exclusions that cost you money
Blocking your own brand name because it "converts anyway", blocking question phrasings because they look like research, blocking mobile because it converted worse last year. Each of these has a real cost that nobody measures because the traffic simply stops appearing in reports.
Do this: Before excluding anything that has ever converted, check its cost per lead rather than its conversion rate.
This is teaching material and our own reading of public documentation, not advice for your specific account. Check anything important with your own specialist before you act on it.