If the plate has mill scale (dark gray coating), then you want to soak it in vinegar overnight and give it a good scrubbing. Otherwise, if the steel is clean, I would take a very simple approach to seasoning it- wipe on an extremely thin layer of oil before baking the first couple times you use it. Unlike cast iron, where you need a thick layer of seasoning for non stick properties, steel only needs to be sealed/protected from the elements.
Btw, it sounds like you won't be shopping for A36 anytime soon, but, before you do, make sure your setup is appropriate for steel. Steel isn't the answer for everyone. If you're cursed with a weak 260C (500F) or less home oven, steel won't do much for you. Also, if your oven doesn't have a top heat source (broiler), then steel isn't recommended there either.