Pour a small amount of oil (maybe a teaspoon or so - I always eyeball it!) onto your cloth.
Wipe the surface of the cutting board, rubbing gently to get the oil everywhere. You will continue to add more oil and rub until the entire surface of the board is glossy and will take in no extra oil. Make sure to get the sides of the cutting board as well!
Make sure to add the oil little by little so that it gets a chance to sink in to the wood.
Now turn the board over the do the back in the same way.