Welcome to the forum.
Just connect the BR 800 to your computer with the USB cable. Turn on. Open SONAR. Check that the BOSS is seen by SONAR. You might have to click the Wave Profiler button under Options>Audio.
Select the BOSS as an input in an audio track.
Mute all the tracks you don't want recorded on the SONAR track.
Arm the track for recording.
Press play on the BR 800.
Adjust the output level of the BOSS so it's not clipping in the SONAR track. Once the level is correct, rewind(?) the BOSS, click the Record button in SONAR, start the BOSS again and the BOSS track will be recorded into the SONAR track.
You can probably record more than one track at a time from the BOSS to SONAR, but the exact routing from the BOSS to SONAR will require some trial and error on your part. And the BOSS may only be able to send a single stereo signal or 2 mono signals at a time, which means you might only be able to record 2 tracks at a time in SONAR. The BOSS manual should help with that info.
Make sure you download the latest drivers and updates for the BOSS and for your operating system:
http://www.bossus.com/gear/productdetails.php?ProductId=1068 You can have SONAR on more than 1 computer as long as you only use 1 at a time and you (the registered user) are the one using it.