Creating Eggnog Snickerdoodle Thumbprint Cookies is straightforward if you follow these simple steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
2. Combine Dry Ingredients: In a medium bowl, whisk together the flour, baking soda, cream of tartar, salt, cinnamon, and nutmeg. Set aside.
3. Cream Butter and Sugar: In a large bowl, beat the softened butter and granulated sugar until light and fluffy, about 2-3 minutes.
4. Add Eggs and Flavorings: Add the eggs, vanilla extract, and eggnog to the butter mixture. Beat until well combined.
5. Incorporate Dry Ingredients: Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined.
6. Form Cookies: Scoop out tablespoon-sized amounts of dough and roll them into balls. Place them on the prepared baking sheets, spaced about 2 inches apart.
7. Create Thumbprint Indentations: Gently press your thumb (or the back of a spoon) into the center of each cookie to create a small indentation.
8. Bake: Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden. The centers may look slightly underbaked; this is normal.
9. Cool: Remove from the oven and let the cookies cool on the baking sheets for about 5 minutes before transferring them to wire racks to cool completely.
10. Prepare the Frosting: In a small bowl, mix together the powdered sugar, softened butter, eggnog, and vanilla extract until smooth and creamy.
11. Fill Cookies: Once the cookies are completely cooled, fill each thumbprint with a dollop of the eggnog frosting.
12. Garnish: Optionally, sprinkle a pinch of ground nutmeg over the frosting for an extra festive touch.

These steps will guide you in creating these incredible cookies with ease.