Work through the method described here: https://www.simple-talk.com/sql/performance/finding-the-causes-of-poor-performance-in-sql-server,-part-1/

Yes, it's out of date, no I haven't got around to updating it with Extended Events. It's still valid, you can use Extended Events in much the same way. Identify what the worst aggregate CPU consumers are, not individual. Once you've identified the worst 3-5 queries, capture the plans from the application using Extended Events, not from SSMS (they can be different) and tune those.

Try out the index suggestions in the plans. They're suggestions and hence need testing, but they can be a useful place to start.

I'd link the video on my overall tuning methodology, but I think I know what kind of reply that'll get.