By default, DR880 is already in auto sync mode, unless you have changed it. You just need to make sure Sonar is sending midi sync correctly.
At the Preference, select Project - MIDI
- check on Transmit MIDI Start/Cont/Clock.
- Optionally check on Transmit MIDI Song Position Pointer (if you are recording DR880 Song - check this, for Pattern recording - leave it off).
- Check on the MIDI Sync port that the DR880 is connected to.
When you start Sonar, DR880 should start automatically. It's better to have 1 empty bar at the DR880 (song or pattern) before recording DR880 (midi or audio). Without the empty bar, sometimes there may be a miss at the very first beat.