Pollux Have a look at the extension tag. Each extension should com with instructions how to install it. If you want to write extensions yourself, have a look at my post here: Pollux
RelatedTitle The long way is to use composer but you can also install Bazaar (https://discuss.flarum.org/d/5151-flagrow-bazaar-the-extension-marketplace/6) Which you simply have to click on it and it's done
