This is a fun technique with a wonderful result. You almost can't do it wrong, because it's just about all freeform and what it looks like at the end is totally up to you. So, if it looks good to you, it's right.
This is how you do it...
Before you begin, lightly sand and wipe clean the piece you are going to work on.
Use oil-base enamel paints for the marble application. Marble comes in many different colors so choose colors that match your decor.
Haphazardly brush on a combination of three to five colors in a small area, creating a patchwork effect.
Carefully dab the paint colors onto one another with loosely crumpled newspaper or a sponge. Do not smudge or smear the colors. Use an up and down motion to achieve a mottled effect. The marbling is more realistic if your movements are spontaneous, so work quickly.
Experiment on a scrap piece of wood before trying this technique on a permanent piece.
Repeat the process until the entire piece is covered.
Allow the paint to dry.
Add veins of color randomly with a thin brush. Use light and heavy strokes, and smooth and jagged ones to create uneven veining.
Apply several coats of high-gloss polyurethane, sanding between coats to finish.