cut across the equator, i.e hold the pinched end towards you and cut down the middle on the long side. do it slowly, otherwise piss will splatter on you.
Like sinophile said, you want to cut longitudinally, meaning cut down the long side. So if you place the kidney on a horizontal table, cut horizontally at the level of entry of the ureter on the opposite long side. Cut all the way through so you get a neat cross section of the inside.