After talking with the pyramid man and making your decision to dig, he will instruct you to find a flat rock, a branch from a tree and grass to create your shovel.
Head left from the pyramid and take a branch from one of the many dead trees. Then find the pile of flat rocks behind one of the diggers and take one. Finally, head back to the cave entrance where you started and take some of the grass that's there. Once you have all 3 ingredients a small dialogue cutscene will begin and you will craft the shovel obtaining this achievement.