I recently recorded a complete video tutorial series on Affinity Designer for Stone River e-Learning. You can check out the entire lesson here. The course teaches everything from illustration, logo design, user interface mock-ups, and exporting for web and mobile devices. However, if you’re just wondering how Affinity Designer can help you mock up mobile user interfaces, here is a tutorial.
You can download the entire step-by-step tutorial and the resource files. You are free to use the artwork provided for personal or commercial use. However you cannot sell or distribute the tutorial.