First you must be sure you have some logs in storedge. Then, during winter, after the mine has opened (the tenth) go to the side of your chicken coop and get a piece of lumber.Put it in your nappsack and make sure you have your hoe. Once you have your piece of lumber, go into the mine before it closes (five o'clock). Using your hoe, dig up one spot of earth. Place your piece of lumber on the spot you hoed and it will stand there covered with snow.

Note: Right after I placed the piece of lumber in the mine I found a Pontna Root and then a Moonlight Stone.