Creating the Apple Crumb Cake is a straightforward process if you follow these simple steps:

1. Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan.
2. Prepare the Apples: In a mixing bowl, toss the diced apples with lemon juice and set aside.
3. Mix Dry Ingredients: In a separate bowl, combine flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon.
4. Cream Butter and Sugar: In another large bowl, beat the softened butter with granulated sugar until light and fluffy.
5. Add Eggs: Incorporate the eggs, one at a time, mixing well after each addition.
6. Add Wet Ingredients: Stir in the buttermilk and vanilla extract to the butter mixture. Mix until combined.
7. Combine Dry and Wet Mixtures: Gradually add the dry ingredients to the wet ingredients, mixing just until blended.
8. Fold in Apples: Carefully fold the prepared apples into the batter until evenly distributed.
9. Prepare Crumb Topping: In a small bowl, combine brown sugar, rolled oats, cinnamon, and melted butter. Mix until crumbly.
10. Pour Batter: Transfer the batter into the prepared baking pan and spread evenly.
11. Add Topping: Sprinkle the crumb mixture generously over the batter.
12. Bake: Place the pan in the preheated oven and bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.
13. Cool: Once baked, allow the cake to cool in the pan for 10-15 minutes before transferring it to a wire rack.

Following these steps will ensure you create a delicious Apple Crumb Cake that everyone will love.