For Bounty Hunters:
Step One: Click 'View Bounty'Here you can see the specific requirements of a bounty, and specific instructions as to how to submit it.
Step Two: Once you have decided that you are going to complete a bounty click ‘Start Work’. This indicates to the poster that you are working on their project.
Step Three: Submit your work. Make sure to include your Ethereum Address
Here is a Video Guide on Posting and Hunting Videos