Walnut ink is very easy to use, and it can be used different ways. But, it always has the same effect - it makes your piece look aged and worn.
To prepare the ink, mix 1 TBLSP of the walnut ink granules with 1 cup of hot water. If you want the ink to be lighter (more of a tea stain), add more water. If you want it thicker (like Indian ink), add less water. The mixture does not need to be refrigerated when you store it. Put a lid on it to keep it from evaporating, and it's good for a long time. Even if it DOES evaporate, no problem, just add some water to dissolve the residue in the bottom of your container and voila - more walnut ink.
To use it....try these different approaches:
Pour the ink into a zip lock bag, put the paper or cloth item you want to stain into the bag and let it absorb the ink until it's as dark as you like. Remove the item and lay flat on a paper towel to dry.
Sponge the walnut ink onto your paper
Use a paint brush or stipple brush to transfer the walnut ink onto your paper
Put the walnut ink in a spray bottle, and squirt it onto your paper to get a spattered design.
Get your paper nice and wet with walnut ink, then sprinkle salt on the water paper, for another interesting effect.
Ink your paper while it's smooth; or crumple your paper before you ink it. Crumpling your paper breaks the paper fibers, and the broken fibers will absorb and hold more ink than the smooth fibers.....so your weathered look will be even more obvious.
It's a debate on whether walnut ink is archival or not. Walnut ink made from actual walnut shells is NOT acid free. However, some manufacturers are not using actual walnuts anymore to make their ink....and they claim that their product IS acid free. The trick is knowing which is which. So, unless your container expressly states that it is acid free, I would recommend that you avoid using walnut ink near a precious photo.