Start by cutting a slice at the size you desire. Pick the one you like - here, I chose one with the grain radiating from one side.
Once you have chosen a piece, clamp it to a desk and start carving grooves in between the grain. I chose to do a shallow groove on one side - you can get some interesting results if you go deeper and on both sides. Experiment!
Using a small, folded piece of sandpaper, proceed to make it as smooth as you like. You can go over it with an electric sander but you run the risk of masking the darker shade on the grain ridges, so I chose to do it by hand.
And there you have it! It's simple and straightforward with a neat, functional result. Have fun!