Drag to Add to this layer or Erase from it. Zoom with the +/− buttons, mouse wheel, or pinch; space-drag (or middle-drag) to pan. Your edits are kept in the export.
Each layer below is one mask. Click a layer to open it large and brush cleanup. Cut the black areas; stack the layers in order and the image rebuilds.
Paint order
Work dark to light: block in the darkest masses first and build up to the highlights. Names are a starting point — mix to taste.