Take your piece of wood and drill a hole in the middle. With a dremmel and a sanding band attached, sand it down to fit your finger.
Keep going with a finer grit sanding band till it's nice and smooth.
Polish the inside and out with a felt cone and some beeswax.
Just melt a piece of beeswax over a candle and work it into the wood with the felt cone. It helps protect the wood and really brings out the color.
Enjoy your new ring!