If you want to create a YouTube channel cover, you first need to select the appropriate Youtube preset size from the templates list - 2560x1440 pixels.
Once you've selected the size, you can start a design from one of the templates listed or create one from scratch and add your own content.
YouTube covers are automatically cropped for mobile by YouTube's mobile responsive function. To make sure your cover will look great on both desktop and mobile, adjust your design, so the main elements fit within a 1546x423 rectangle in the middle. The easiest way is to use a size stencil.
When you've finished your creation, download it to your computer and upload it as your YouTube channel art.
*You can download the YouTube size stencil here.*