First of all, you have to fill out a survey which is available on the brand's website. After that, you have to wait for their reply, and they'll let you know if you are eligible for the job or not. Companies receive a lot of such requests and applications, so they need to pick the best among them. The selection depends on your experience and your affiliation with the particular niche. For instance, a tech company would prefer testers with some prior tech knowledge. Otherwise, the review will not be considered authentic. If you are eligible, you'll be approved for the job. They'll send you the product, and then you just have to review the product in the required duration.