There are lots of post saying it's hard to find, due to it's not-so-obvious location, and the fact that GPS directions arent going to be consistent in Venice.
Get to Campo Manin. It's a 5 minute walk from St. Mark's Square. Exit the square on the East side from the alley right in the middle. Walk this alley until you need to turn and take a left. Then take your next right.
The staircase is not in the middle of a big square so it will sneak up on you and surprise you, which is part of it's charm. We stayed in a place within it's square, actually attached to the building. That was quite amazing. It is very much worth a visit.