In Blender: File → Export → glTF 2.0, pick glTF Binary (.glb). 1 Blender unit = 1 stud; put the bottom of the hat at the centre. Max 3 MB — use the Decimate modifier if it's bigger.

Draw on the template — the faint boxes show where each side of the body goes (they won't be on your clothes). Use 🪣 Fill to colour a whole box in one tap.

Works exactly like Roblox classic clothing: a 585 × 559 PNG. Paint over the blank template, keep see-through bits see-through and they'll show your skin.