Click the Extensions button on the campaign or ad group row (or select items in bulk and use Extensions in the bottom bar). In the modal, pick a level under Apply to: Entire account, Campaign or Ad group. Then go through the Sitelinks, Callouts and Snippets tabs. Sitelinks need link text (up to 25 characters), a final URL and optional descriptions (up to 35 characters). Callouts accept up to 25 characters. Snippets use an official Google header (such as Services, Brands or Types) and values of up to 25 characters each. Google recommends at least 4 sitelinks, and at least 2 are needed for them to start showing. The table in the modal lists the existing extensions at the chosen level, with Edit and Remove options.