Which part din’t you understand?
You install the component and to change the homepage for everyone you can use the custom default homepage setting of the component. You enter a relative URL, so if you forum was https://forum.example.com and the category you want as your homepage is at https://forum.example.com/c/Kategorie/4 then you put /c/Kategorie/4 into the field, save it and use the preview link to check whether it’s working. If it’s working you add the component to your theme and refresh the site so the changes you made to your theme are loaded.