STEP FOUR So the next big step is to add the black gouache streaks. Many people use feathers but I just used a round 01 paintbrush and a little bit of water and dragged it across from one end of the table to the other, frequently lifting up to add breaks in the lines (think dotted lines.) I added a few veins here and there as needed stretching out from the original line. Then I added extra thicker areas on each end made it more realistic. This looks like absolute crap at first but I promise it is worth it. You can add as many as you feel necessary.
As the gouache begins to dry, you will want to sponge over with the mixed acrylic colors very faintly so that you don't remove all of the black. Then you sponge and drag and sponge and drag, until you achieve a more marbled look. The beauty of it is if you mess up, just keep layering! If the paints are going on too thick, adding a bit of water to your brush will give it more transculency. The details will eventually work themselves out as they dry.
I made a higher-contrast layer and ended up nixing it because it was too much and went back to a more low-key version like before. Through trial and error I did something like this three times in an evening before the layers ended up exactly the way I wanted them. I then let it dry overnight and (optional step) sanded it down with a fine grit sandpaper where any excess paint resided.