Before each basho the Japan Sumo Association publishes the banzuke, the ranking sheet that says where every wrestler stands. You place the top division's 42 men on it first.
Each man scores 5 if you put him at his exact rank and side, and one point less for each half-rank you were out. East and West of the same rank are a half-rank apart. A man you left out of the top division scores nothing.
Within a named rank the number doesn't count, so Ozeki 1 East and Ozeki 2 East are the same place. The named ranks are Yokozuna, Ozeki, Sekiwake and Komusubi.
Banzuke's own guess goes on the table too. It's a rule of thumb from the results: beat it. More exact picks break a tie on points, and players still level share a place.