Google is a top notch search engine and quality assurance is something the company deems important. Don't be scammed by all of the fakes out there claiming to have an in with Google. A few independent companies hire work at home independent contractors to act as Quality Raters to rate sites on various levels.
Leapforce, Work Force Logic and Lionbridge are the current companies that contract with Google to hire raters. Each has different requirements for their work at home raters.
Research the three companies and what opportunities they offer. Decide whether to apply to one or all of the companies. Apply for a new Gmail account or use an existing Gmail account to apply to the positions you are interested in. Read the instructions and follow the directions completely. If they ask for your resume to be in html format, don't submit a word doc. They will not take a look at it, they do get a lot of applications and don't want to hire people who cannot follow simple instructions.
It may take a while to hear back about your application. It appears that these companies hire at random times and might even pick random batches of applications to review. Apply once a week or so until you hear back whether you are accepted or rejected.
When you are accepted you will complete training and exams to become a rater with Google. If you pass the exam you will be hired on as part-time, work at home Ad Rater or Quality Rater for Google.
Follow instructions. Proofread and polish your resume. Continue to apply until you hear back.
Do your due diligence. Don't be scammed.