Another method you could try is get a fairly stiff brush (at least 1"), the cheaper the better because you want it to separate a bit. Even bash it down into something hard to mess up the bristles
Now, spray a flat primer coat of white on to whatever you want to paint, followed by a thick coat of yellow brushed on. The trick is not to let the paint settle, and have your strokes the full length of the piece you're painting. Normally I brush it on sloppily, leave it for a couple of minutes, brush for a minute, leave for a minute, brush for a minute etc.
As the paint starts to dry, you'll see the "grain" appearing (be careful when the paint starts to thicken, it's very easy to brush too heavily and end up with areas too heavy in paint and others too light)
After that coat has dried, follow up more coats in exactly the same way with progressively darker colours. Light brown, medium brown, dark brown should be enough, but you might wanna throw some black in there while the last coat is drying to give it an aged look. Alternatively, weather it after it's dried.
If you do it right, it should look something like this
Good luck!