This isn't the only way to create annotations, but this is a simple method for quick notes.
Log into your YouTube account and navigate to the watch page of the video you want to annotate.
Play the video to where you want to start your annotation, and then click the plus sign on the bottom left of your video.
If you don't see the link to add an annotation, make sure you are logged in to the correct YouTube account, and make sure the Annotations Editor button just above the video is toggled on.
