A black gown with full sleeves shows an open upper shoulder line and closer torso with visible vertical divisions. Side sleeves begin lower and form broader rounded areas. The dark skirt continues longer below, with a horizontal division visible towards the lower section. Hands sit before the middle. The window setting mainly helps compare quieter neck area and fuller sleeves without revealing complete rear construction or establishing the actual arrangement of every inner layer.
Try a similar upper shape with relaxed arms and underwear first. Reach for a glass and sit before rearranging sleeves. Compare volume with a narrower sleeve arrangement if still undecided. Keep the dark base unchanged so that shoulder and arm distribution becomes clearer. You can enjoy black in either shape rather than changing colour with every fitting question.
Add hair and jewellery individually afterwards. Have upper attachments and back shown on the actual model. Try planned outerwear, remove it and inspect sleeves again. You may enjoy the open neck area while seeking another skirt width. Material, internal support and possible alterations need information about the real garment rather than answers from this picture alone.