Very simple.
First you get a bottle of clear mineral oil from your local drug or grocery store.
If your cutting board is new, just slather the stuff on all surfaces so it's obviously oily and let it soak a while. If it all absorbs (unlikely for a hardwood board) apply a second coat. Wipe off the excess with paper towels. You don't have to be too thorough, it's approved for human consumption.
If your cutting board is used, first scrub it with cleanser (Comet or similar) to remove vegetable and animal oils which can become rancid. Dry thoroughly, preferably in the sun. proceed as for a new board.
When the board has been scrubbed clean a lot and is looking dry, just wipe some oil onto the working surface.
This oil is excellent for wooden knife and cleaver handles also.
