This puts a study piece on your shoulders using your camera. It is a silhouette study, not a fitting — it will not tell you a size, and it does not know how the fabric would fall on you.
Wearing
It works best facing the camera, from about the waist up, with your whole shoulders in frame; visible hips help with leaning. Each piece is a 3D garment with a skeleton: it turns as you turn, and its sleeves follow your arms. You can change piece without turning the camera off.
Starting…
A photograph is read in this tab and never uploaded, exactly like the camera. It is fitted once and held still, so it can be solved carefully instead of thirty times a second — which is the only reason it comes out better. It is the same 3D piece either way; it is not a generated photograph of you wearing one. The piece is a render, not a photograph. There is no cloth simulation or size recommendation. The tee has authored folds that change with arm pose. The camera estimates which parts of your head and exposed arms pass in front of the piece; edges and depth can be wrong, especially when you turn. Sleeve length and shoulder width are approximated from visible landmarks — so it needs to see your shoulders, and will say so when it cannot. The piece is trimmed to your own outline and takes the colour and brightness of the room you are in, and outerwear is widened so it contains you rather than sitting inside you. That outline is of whatever you already have on, not of your body, so a bulky coat underneath will read as a bulkier you. It will not drape the way the real thing would. Treat it as a silhouette study of proportion.