Simply upload the reference image and you're all set.
For the best results, use a high-quality image. Ideally, choose a medium shot or a full-body view of the subject so the AI has a strong reference for the pose and overall structure. It will still work with other types of images, such as close-ups, but the AI may improvise on unseen areas, which can sometimes lead to less ideal results.
In the subscribers section of my profile, I've also released the same app with a 20-second duration.