You select Path tool.
Position the edges on the skin. Then you start clicking on the lines and pulling them so they start to bend.
Once you're happy with the way the curves look, you click on "Path to Selection" button in the toolbar.
Once you have it selected and are on a proper layer, fill the selection, either with bucket fill or Ctrl+,
If you want a 2 tone curve, you have to deselect some areas that you do not want to change, create a new layer(or do it on the same, although, better to create a new one) and fill that new selection with another color. Deselection can be done by pressing down Ctrl key and using one of the selection tools(don't know if it works with path tho). If you want a line(curved or not) that has a white border all around, you first fill white and then can use Selection shrink from the menus to shring the selection by X pixels and once shrunk just fill the new selection with another color.
It's basically just practice and patience.