The best time to ask is right after the job, while the customer is still pleased with your work. Don't wait a week.
If your account has the Google Profile section in the left menu, open the Reviews tab there. It has "get more reviews" tools that make asking a one-tap job. You send the customer a link they can tap to leave a Google review.
If you don't have that section, you can still do this by hand. Text the customer your Google review link directly, and they can tap it to leave a review.
Reviews people leave you can then be shown right on your website. You turn that on in the Testimonials tab, under the Customers heading.
Why this matters
Good reviews are what make the next customer pick you over the other guy. Asking at the right moment is the easiest way to get them.
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