Customer reviews and testimonials should be added to your social media and listing site profiles, such as Google My Business, Angie's List, Facebook, etc.
The best places for customers to leave reviews are on websites that show your average star rating in search results, instead of only on your listing profile. Which is great if you have all 5 star reviews, but not so good if your average reviews are lower than 4 (find out how to deal with negative reviews). Google My Business and Yelp are two of the most important online listing websites that are very popular and have this capability. But there are several other listing sites that have this capability as well.
In order for someone to leave an online review, they need to create an account with the listing website, and then log in to leave the review. Which is why Google My Business is the most popular site, because you simply log in with a Gmail email address, and Gmail is a very popular email service.
The downside to allowing customers to leave reviews on listings websites is that you cannot delete the review. If the review is not positive, in some cases all you can do is reply to it. This is why pre-screening customers before you send them a link to the review site is important. The upside is that you can copy these reviews to your website's Reviews page, with a link back to the page where it was originally submitted, which adds even more credibility to the review.
If you have a Facebook page, customers can leave recommendations on the page. The person leaving the recommendation must have a Facebook account in order to post on the page. You can ask the customer to "like" the page while they're on it (the more "likes" your Facebook page has, the better). The down side to allowing recommendations on your Facebook page is that unhappy customers can "not recommend" your company. However, Facebook allows you to hide ALL recommendations as well as prohibit recommendations completely, so if you get too many "not recommended"'s, you at least have an out (although if you get a lot of "not recommended"'s, the better course of action is to find out what you can do to improve your service).
Just ask. If customers are happy with your work, they will generally be willing to provide you with a review. Make sure to request the review as soon as possible, ideally in person, after the work has been completed, while it's fresh in your customer's mind. Otherwise, you can send your customer a text or email requesting the review after the job is complete.
If you mail follow-up letters to your customers, mention the review request again. If you don't get a review within 2-3 weeks, it's okay to follow up with a gentle reminder. You could also provide an incentive for a review, such as a $20 gift certificate, especially if you don't already have a lot of reviews on your website or Facebook page. But keep in mind that offering a reward and incentive only for "positive" reviews is unethical!
If you don't have the time to request and gather reviews from your customers, you can hire a company that provides online customer review services.