Following the path from the top it goes round, crosses over itself (bottom left), goes around again and passes under itself before going back up again. The paper sketch was easy to make.
However, in Illustrator I expected to create this with a single path and an outline of that path. With a single path I ran into the problem that it could never pass over itself. The boundaries would then disappear:
Now I am wondering how I can recreate this image (or any self-crossing path) using a more flexible method which allows for easy editing afterwards.