Select "Get Add-ons" in the "Add-ons" menu to navigate to the Google Docs Add-ons store. Some of the most popular add-ons will be listed on the front page, but you can also browse by categories like Education, Business Tools, and Productivity. If you don’t see what you want, try a keyword search.
Once you’ve installed an add-on, you can access it from the "Add-ons" menu. It’s worth noting, though, that most add-ons come from third-party developers and, while some add-ons are great productivity boosters, some may not work as advertised.