The green shoot in the centre is done using a cardboard stencil: you cut a stencil with a slight downwards taper (i.e. bigger at the bottom), tape it onto the box, fill it with paint and then pull the stencil off carefully while it's wet. This bit was done first. Leave to dry before the next step.
The marble effect is done by putting down a fairly thin layer of one colour (yellow in this case), then using the side of a flat brush to pick up the surface into a ripple pattern. You leave this to dry, then put a thick layer of the other colour (brown) over the top. Leave the whole thing to dry (ideally for a full day in a warm place), then carefully sand it flat with wet and dry sandpaper (used wet) until the bottom layer shows through. You will need several grades of paper, working up from coarse to really fine (I used from 100 to 1200 grit).
You may need to spend a while touching up small patches where you've gone through the paint, and sanding them back to a smooth surface.
Finally, varnish the surface with liquid acrylic varnish. I thinned this a bit with water to reduce brush marks.
The pictures show a few stages of the painting; hope you like them and find something cool to do with the technique.