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

1. Preheat Oven: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
2. Mix Dry Ingredients: In a large bowl, whisk together the flour, cream of tartar, baking soda, salt, cinnamon, and nutmeg until well combined.
3. Cream Butter and Sugar: In another bowl, beat the softened butter and granulated sugar together until light and fluffy.
4. Add Eggs and Vanilla: Beat in the eggs one at a time, followed by the vanilla extract. Mix until fully incorporated.
5. Combine Mixtures: Gradually add the dry ingredients to the butter mixture, mixing until just combined.
6. Form Cookies: Scoop tablespoon-sized balls of dough and roll them in the cinnamon-sugar mixture.
7. Create Thumbprints: Place the dough balls on the prepared baking sheets. Use your thumb to create an indentation in the center of each cookie.
8. Bake: Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden. Remove from the oven and let cool on the baking sheets for a few minutes.
9. Prepare Icing: In a small bowl, mix powdered sugar and eggnog until smooth to create the icing.
10. Fill Cookies: Once cooled, drizzle the eggnog icing into the thumbprint of each cookie.

These steps will guide you in creating these incredible cookies effortlessly.