I don't know the torque spec but the boarding step is a very simple device. The step itself fits up into the ski rail there is a notch under there for it) and connects to the hull of the ski with 2 screws. So simply fit the step into the only place it can go (right rear), drill 2 holes, install the 2 screws until they bottom out against the step bracket. Too much torque and you'll start stripping the plastic hull, so hand tight/snug.
Put some black silicone in the drilled holes first to form a seal for the screws.