Paint the board with wood primer. Allow to dry, then apply a single coat of white semi-gloss paint. Leave to dry overnight.
Prepare your materials. Pour white spirit into a shallow dish; an old saucer is ideal. Squeeze out the black or brown oil paint into a similar dish. Place the board on a table in front of you. Place the second tin of oil-based paint within easy reach, together with the two saucers, feathers, brushes and rags.
Paint the board with a coat of the second color. Working quickly while the paint is wet, dip a feather into white spirit and gently drag it through the wet paint. Start a corner of the board farthest away from you and bring the feather toward your body, diagonally. Move the feather in such a way that it lifts the paint, leaving a white zig-zag "vein." Don't go from corner to corner; stop and begin another line, always working in the same direction. Wipe the feather on a clean rag occasionally before dipping it back into white spirit. Continue until you have made several white veins in this way. Use an artist's brush to lift out thicker lines if you wish.
Change to a clean feather and dip the soft side into the black or brown oil paint. Drag the feather across the board as before. Allow dark veins to cross and merge with the white ones. Don't overdo the dark lines. Have some lines split into two like forked lightning and a few others going in the opposite direction. Go back to the white spirit doused feather and drag that one through the dark lines. Play around to see what effects you can achieve.
Sweep the soft decorator's brush very lightly across the surface. Be very delicate about this; barely allow the bristles to touch the paint. Move it from side to side until the painted lines soften and merge. At this point, if you were working on an actual project, you would leave the finish to dry completely before applying a protective coat of high-gloss varnish. However, if you want to practice some more, wipe the board clean with a cloth dampened with white spirit. You will be left with the white painted surface you started with.
Follow instructions for an actual project once you are comfortable with your marbling abilities.