Pretty slick stuff! This very short vid is well worth watching:

With Sketch mode, we’re introducing a new palette of tools and guides that let you start taking control of your skybox generations. Want a castle in the distance? Sketch it out, specify a castle in your prompt and hit generate to watch as your scribbles influence your skybox. If you don’t get what you want the first time, your sketch sticks around to try a new style or prompt from – or switch to Remix mode to give that castle a new look!