Three routes: paste real quotes as styled text (free, fast. With name and job type for credibility), embed a reviews widget that syncs your Google reviews automatically ($10–30/month tools), or add Review schema markup so your stars can appear in search results.
Placement beats quantity: one specific relevant review beside each service page's call-to-action outsells a fifty-review wall on a testimonials page nobody visits. Match the review to the page. The water-heater review on the water-heater page.
The honesty rules: real reviews, verbatim (typos are credibility), never invented, and schema only marks up reviews genuinely collected. Faked rich-snippet stars are a penalty Google actually enforces.