Select a long, stiff bird feather that is about 1 inch wide at its widest point and in good condition. Pheasant feathers work well for veining.
Snip the spine of the feather about 3/4 to 1 inch down from the tip without cutting the sides. This will leave you a V-shaped tip to your feather.
Dip the two points of the V in your paint.
Hold the base of the feather's spine gently between your thumb and first two fingers.
Drag the feather across your faux stone surface so the tips line up with each other to create a fine line, and then roll the spine just a little between your thumb and fingers occasionally to make the two points of the V flare out slightly, thickening your line. Your loose control over the feather should keep your line from becoming unnaturally straight.
Lighten your pressure on the feather as you begin and end each of your desired line lengths so both ends of each vein are the finest line possible.
Draw veins with your feather at various points on your stone, making sure you do not create patterns that are too similar to each other.
Brush across the length of the lines with the side of your feather as each line becomes partially dry. This softens the edges to create more realistic veins.