This is the scary part.

Using black Indian ink, paint over the entire picture.

The aim is to lay a skin of Indian ink on top of the painted image.

Next, leave it for several hours until it is completely dry.

Do not worry when your image disappears under the black ink.

It will reappear during the next stage.

Note: Try to cover you picture in a single coat. Use soft, even brushstrokes to liberally apply the ink. Continued brushing over one area will start to dissolve the paint underneath and, once dry, you will find it impossible to remove the ink from that area.