Follow these simple steps to bake your lemon poppy seed muffins:

1. Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners or lightly grease the cups.
2. Prepare the dry ingredients: In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and poppy seeds until well combined.
3. Zest and juice the lemons: Using a microplane or fine grater, zest the lemons, ensuring not to include the bitter white pith. Then, juice the lemons and set both the zest and juice aside.
4. Mix the wet ingredients: In a separate bowl, combine the melted butter, eggs, Greek yogurt, vanilla extract, lemon zest, and lemon juice. Whisk until all ingredients are well incorporated.
5. Combine wet and dry ingredients: Pour the wet ingredients into the bowl with the dry ingredients. Gently fold the mixture together using a spatula or wooden spoon until just combined. Be careful not to overmix, as this can result in dense muffins.
6. Add milk for consistency: If the batter seems too thick, add the milk a little at a time until you achieve a smooth consistency. The batter should be thick but easily scoopable.
7. Fill the muffin cups: Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
8. Bake the muffins: Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
9. Cool the muffins: Remove the muffins from the oven and allow them to cool in the tin for 5 minutes. Then transfer them to a wire rack to cool completely.