ishak where are you stuck and what error message do you see if any ? I don't think there are many steps:
1) Install extension via composer with the command given on the extension page
2) Enable extension
3) Profit ?
Flagrow Sitemap doesn't even have any setting, the sitemap will be available as soon as enabled.
EDIT: I just realized there's also the nginx fix below the composer command, but you only need to do that if you're using nginx and encounter the issue where nginx refuses to let Flarum answer for
/sitemap.xml, which depends on hosting. Just install & enable the extension and see if it works 😉