Surfing Bonus credits are credits you earn in addition to your normal ratio. It fluctuates based on various items like completing the checklist, the number of members surfing, etc. You can view the bonuses available anytime by clicking the Surfing Bonus % on the surfbar (or click here).

The bonus credits are automatically earned the same way you earn normal credits from your ratio. You don't need to do anything special. If you want to be sure, turn auto assign off and watch your un-assigned credit balance. You should see more than your ratio being added if your surfing bonus is greater than 0%.