Customizing your One Channel Layout in YouTube can be done in a few simple steps. First, sign in to your YouTube account and click on your profile picture at the top right corner of the screen. From the drop-down menu, select "Your Channel". Once you're on your channel's page, click on the "Customize Channel" button which is located near the top right of the screen.

In the customization page, you can change your channel's layout, branding, and basic info. To change the layout, click on the "Layout" tab. Here, you can add a channel trailer, featured video for returning subscribers, and organize your video and playlist sections. To change your channel's branding, click on the "Branding" tab. Here, you can add a profile picture, banner image, and video watermark. Lastly, to change your channel's basic info, click on the "Basic Info" tab. Here, you can add a channel description, email address, and links to your websites and social media.