Instructions
Proceed through the game until you reach level 26. You gain levels by performing farming tasks such as weeding, watering and tending the soil.
Go to town and select "Willy Builder." Select "Mine" from the construction options. Place it on your property, ideally far away from your garden so that it doesn't get in the way.
Return to the village menu and select "Arfur's Inn." Speak with a Diggerling to hire him. Afterward, he gives you a contract. You can hire up to four Diggerlings at once.
Drop the contract next to your mine. Sometime over the next 24 game hours, he'll arrive and start working. When his shift is over, you have to return to Arfur's Inn and hire him again.