Mobile sitemaps use the same XML format as regular sitemaps but prioritize mobile-friendly URLs. Use the mobile:mobile tag to indicate pages optimized for mobile devices.
If you use separate mobile URLs like m.example.com list them in the sitemap. For responsive sites your regular sitemap is sufficient since the same URL serves both.
Use the mobile namespace to annotate mobile pages: xmlns:mobile=http://schemas.google.com/mobile/crawlers. Add mobile:mobile element inside each URL entry.
Desktop pages should have rel=canonical pointing to themselves and rel=alternate pointing to mobile URLs. Mobile pages should have rel=canonical pointing to desktop URLs.
Submit your mobile sitemap in Search Console. Monitor the mobile usability report for any issues with your mobile pages.