A very small gouge is used to cut a groove along the outside border of the moon and tree. This will act as a “stop cut” that will help prevent the interior of the tree and moon being cut into while the waste material is being removed.
A medium-sized gouge is used to remove the waste material in the more narrow passages, such as those between the limbs and branches. A larger gouge is used in the more open areas, such as those around the tree trunk.
Since the grain is parallel to most of the edges, the wood can easily be shaved away in the direction of the grain with very little cross-grain cutting.
This process requires a good deal of patience to slowly shave away the waste wood to achieve the desired depth. Doing so keeps all the surfaces fairly level, ensuring the depth remains balanced on all sides.
Once an even layer is removed, the small gouge is used to cut another groove around the outline and the shaving process starts again. You can see the gouges in the photo.
Of course, the entire process could be done faster with small power carving tools. But, holding the focus necessary to complete the task with hand tools is a meditative pleasure and one of the things I enjoy most about the hobby.