Joanna, One method that has worked well for me on steel is to use a
spray bottle with vinegar for a nice even application. Each type of
vinegar (white, wine, rice, balsamic, etc.) produces a slightly
different patina color and texture. I would recommend that you test
each of them on a labelled scrap to see what works for you. It
should take you up to 3 days for the first layer of patina to fully
develop -- after that, successive applications of the vinegar (using
a sponge or sprayer) will deepen the patina much more quickly. Allow
to air dry between applications of vinegar.
Keep in mind, too, that the steel's surface needs to be "roughed up"
in a best case scenario -- steel wool or sandpaper are best. And it
must be clean before starting -- clean it well with denatured alcohol
before your first spraying to avoid unevenness.