Spray the paper with water and the ink will start to spread. To build up a more dense effect, carefully re-crumple the paper and repeat the process. The paper will be fragile when wet, therefore handle it with care - particularly when crumpling and uncrumpling.
The picture shows how the paper looks when it has first been rubbed with ink, compared with how it looks after it has been sprayed with water.
See the next page for a variation on this technique.