Pick a schema type, fill in the fields, and copy a valid JSON-LD block into the head of your page. Article, FAQ, Product, Organization, LocalBusiness, HowTo and BreadcrumbList.
Choose the schema type that matches the page: Article for posts, Product for a product page, LocalBusiness for a business with an address, FAQ for a page with questions and answers. Fill in the name, description and URL, add an image and an author if you have them, and press Generate. Copy the script block into the head of the page, or into the custom code field of your CMS, then check it with Google's Rich Results Test.
Structured data tells Google what a page is in a format it does not have to guess at, and some rich results, such as review stars, FAQ dropdowns and breadcrumbs, only appear when it is present. Pages with valid schema take more space in the results and get more clicks at the same position. JSON-LD is the format Google recommends, and it lives in one script tag that does not touch your visible HTML.
Anywhere in the HTML, but the head is the convention. WordPress users can paste it into a header scripts plugin or their SEO plugin's custom schema field. Shopify and Webflow have a custom code area in the page settings.
Article, or BlogPosting if your CMS offers it. Add FAQPage as a second block only if the post has a real question-and-answer section on the page.
It is not a direct ranking factor. It changes how the result looks and how well Google understands the page, and both of those change clicks, which is what you are after.