I have heard about this technique for years - long before I even knew what a blog was...and finally got around to trying it. I think I might be in love! Staining my wood (that I use to make signs) with a mixture of cider vinegar and steel wood is so easy, cheap and effective! I am redoing my fireplace and hope to use this method on the new mantel. This method is very easy and requires very few ingredients...don't overthink it, but do let your creativity and the beauty of this stain inspire you! Be sure to protect your work surface and to wear gloves to protect your hands!
NOTE/EDIT: Wood reacts differently to different treatments and in different environments - even wood I purchase from the same place at the same time does not always take the stain the same. Another factor is how long you let the mixture sit/stew - generally the longer it sits, the darker the color. I recently moved to Texas and this stain reacts completely different here than it did in California. If possible test the stain on scrap wood first.