After removing the burned on grease and rust, you are ready to season the piece. Put the pan in the oven to warm it. Remove it and apply shortening. I prefer solid Crisco. Pam spray also will work.
Some people prefer lard or bacon fat.
Put it in the oven at 225 degrees for half an hour. Remove it and wipe it almost dry. You don't want any pooling of the shortening. Place it back in the oven for another half hour. The initial seasoning should be accomplished at this point.