Step by step

As you can see, the panel here is untouched. Before we begin, we need brown oil (any other color would work, depending on the surface color), a semi-wide brush (not the softest type), and a toothpick. The surface must be either satin or gloss, preferably by an acrylic varnish, like Vallejo (I prefer their varnishes, they are a treat to work with!). 

The oil must be undiluted, so basically just get some onto a piece of cardboard. As you can see, I applied dots by a toothpick. Note that it is waaaaay too much, much less is sufficient enough. 

The dots are blurred with the brush. The brush is also dry, and I use a smudging motion to achieve this. Do NOT use dilutant at this step! 

Removing the excess is very easy, just use a Q-tip (ear cleaner, as its called here), without dilutant. If you decide to start again, a wet Q-tip will remove the oil completely. 

And the finished underside. Hope you liked it!