I'm no artist either. That's the beauty of this method. You don't have to do anything. Just drag the paintbrush and your woodgrain magically appears! Make short strokes and that's how you get the variation in color. Believe me, there is absolutely nothing artistic about this. I actually discovered it by accident. LOL
You can get a chip brush at any home improvement store. It's just a cheap paintbrush. I use a 2-inch wide brush, but I use it sideways.
As long as you're in the paint department at the home improvement store, you can usually find a woodgrain tool. It's really wide, though. It's more trouble than it's worth, I think.
I know you don't have time to order anything, but there are woodgrain rollers (rolling pins) available for fondant, for future reference. Still, I think the paint looks better!