Installed a new bathroom on main level with shower stall. How do I connect the drain to a 3 1/2" clean-out on a cast iron pipe?

Installed a new bathroom on main level with shower stall. How do I connect the drain to a 3 1/2" clean-out on a cast iron pipe?
Not enough information. 2" trap and drain line on the shower? This clean out. Is it a clean out tee on its back? Also, this 3 1/2 " measure. Where did you get that from? A hub or a bald end? A picture would be nice. Regards, Tom