On the Video Manager, select videos you want to monetize by checking the box next to each video. You can currently select up to ten videos at the same time. In the Actions drop-down menu, select Monetize.
Finally, you’ll see a confirmation that your videos were monetized:
This update will be available to all U.S.-based partners initially, but we’re rolling it out globally over the coming months. You can also find others ways that YouTube can help you discover your voice and improve your skills, build larger audiences, and monetize your content at the YouTube Creator Playbook 2.