You start by putting a bunch of dots on your paper. Then begin making lines across your page horizontally, but if you run into a dot, you must jump over it. I remember doing this as a kid and loving it! The first sample below is done with an ink pen (it is a little difficult to see in this photo - sorry!)
Here is one done with fine tip Crayola markers and each time I hit a dot I began with an alternating color. The photo is cropped to show you a closeup of it. You don't have to keep super even lines - it adds interest if you don't. AND you can adjust if you want your tunnels to grow or shrink based on how you jump the next row.
You can see how it looks like termite mounds in one direction and tunnels in the other! Op Art 101.
If you want to see some real Op Art, look up Victor Visarely and Bridget Riley.