Luanna, I think the steps to take to prepare steel plate really boil down to the condition of the plate you buy. Mary Ann's plate was in pretty good condition, so she didn't have to take it down to the metal with a long vinegar soak. If you have a good amount of rust, though, or the mill scale keeps coming off on your hands after multiple washings, then a couple days in vinegar is a good idea. On mine, the mill scale seemed very stable and durable, almost like a powder coating, so I left it on.
As far as seasoning goes... from a non stick, food safety or anti-corrosive perspective, you don't really need it. If you've removed all the mill scale and are down to bare metal, an infrared thermometer will tend to have issues taking readings with something that reflective, so an extremely thin layer of oil (any oil) to provide a tiny amount of color will give you more reliable IR thermometer readings, but that's the only reason I can see for seasoning.