Make sure all the edges are smooth before you leave. Take a cotton ball and wipe the edges, if the ball sticks or leaves strings, ask for it to be redone. Especially if they sheared it, water jet is better but not perfect
Wipe the plate with any alcohol that you have (denatured, rubbing, vodka, whatever...) This will remove the 'mill scale'
Spray/rub the plate with any oil that you have (Pam, olive, vegetable, Crisco) and wipe it off. Fire the oven to 400 or so, recoat the plate with oil and bake it for an hour, turn the oven off and let it cool down on its own. Put a cookie sheet under it in the oven in case it drips. Repeat the oil/ heat cycle until it is clean or your just tired of doing it.
Never wash the plate with water, and never scratch it. Use a plastic scrubby to clean it. The oil can be sticky on the plate after it is fired so use a plastic bag or wax paper to store it in. The oil will protect the steel from rusting.