300 hours PIC and 6 months as a CA. Try to get a couple of rec letters from current check airmen and send in a resume to the email specified when they post an opening. It's not seniority based, some get in with their first app and could be very junior while some take a few attempts. There is a networking component and having a good training record helps as well.

After selected you'll be guided through some ground and sim training followed by 8 legs of flying and observations. You'll finish sitting right seat giving a line check during a CA fed ride while the FAA observes you. Takes about a month to complete everything.