Open your account and check five things in order: whether conversions are being counted, which search terms trigger your ads, what you spend per result, whether your budget is being used where it works, and whether your ads send people to the right page. Most wasted spend shows up in the first two.
Are conversions being counted correctly?
Check that each action you care about, such as a call, a form or a sale, is set up as a conversion and has recorded something recently. Without that, every other number in the account is a guess.
Look at the status of each conversion action. A status that says conversions have stopped recording usually means a broken tag or a setup error.
Which search terms are triggering my ads?
Open the search terms report and read the actual searches that led to clicks. Any term that has nothing to do with what you sell is money lost, and you can block it by adding it as a negative keyword.
What am I paying for each result?
Divide what you spent by the number of conversions over the same dates. If one campaign costs several times more per result than another, that campaign is the first place to look.
Are my keywords and ads still relevant?
Keep keywords that bring people who convert, pause ones that spend and never convert, and make sure each ad says what the page behind it delivers.
Source: Google Ads Help: Improve your keywords for the Search Network
Does my budget go where results come from?
Compare spend with conversions for each campaign and move budget from the ones that do not produce towards the ones that do. Do this in small steps and wait for a few weeks of data before judging a change.