As others have said, you can pick any video, start watching and you'll just be "up-to-date". Having said that, one of the first videos I watched from start to finish and found easy to follow was the one about implementing the `pledge` syscall https://www.youtube.com/watch?v=-a5hLBuW6tY. There's also an accompanying blog post https://awesomekling.github.io/pledge-and-unveil-in-Serenity...