If you smash the walnut hulls with hammer, or grind them in a meat grinder, you can pour boiling water over them, and you will get more stain out of them quicker. You will probably need to reduce the liquid by boiling it after you have let the stuff steep for a month or so, to concentrate it. Use a coffee filter over a old can to filter out the liquid from the hulls. Boil the liquid to reduce it, and then run it through both a strainer, and a coffee filter or two to get the fines out. What you will have left is a walnut stain liquid that should give you a good dark stain with a couple of coats. If its not dark enough, add more walnut hulls. The stuff lasts forever in a glass jar, so save it. I have seen it used to stain wooden boxes for gear at my black powder club, and those boxes came out looking very good and dark. You can put a little vinegar, to make the solution acidic, into the mixture when you start out. That will also help pull the oils out of the hulls to make the stain. A cup to a gallon of water should be enough. Any vinegar will do, white or red. You can also throw a couple of tablespoons of salt into the mix as that will also help draw the oils out of the hulls. The vinegar will boil off when you reduce the mix. The salt should not hurt the mixture at all.