How do you create a baking recipe?
I agree with others here that there are basically three ways to create your own recipes:
-magically/by instinct, if you have years of experience or special talents
-by changing/adapting existing recipes
-by learning bakers' percentages and understanding your ingredients thoroughly
And, I guess, there is a fourth way: Close your eyes and guess, which could possibly speed up the experience factor, but will result in more failures than the other methods!
As others have pointed out, there are certain ratios to maintain in cake baking. The book "Bakewise" by Shirley Corriher is, indeed excellent for this.
However, if you don't want to buy the book or want to get started right away, Here are some articles that will help. Two are by Corriher and the last one is on the site baking911.com. These particular articles are on cakes but other articles are available if you search for them.
I see from your profile that you have asked a lot of questions about different kinds of baked goods, from macarons to rice crispy bars.
I occurs to me that if you want baking recipes that are quite novel, you need to take a lot of small steps. Start with known recipes that work, and try some variations. Many won't work, but a few might be an improvement (or more to your liking). Record those, and keep experimenting.
Have you watched or read ATK? It's apparent that they do a lot of research, looking at many variations. Then they test many times. Some improvements are intentional, some are accidental.
Which are you most interesting in: writing, creating something new, or going beyond the existing recipes?
Particular in baking, all recipes are variations on existing ones. Are you happy with the recipes that you use? Do you ever make changes to suit your tastes, or just to try something different.
What, in particular, are you baking? Innovation in bread baking is quite different from innovation in cakes or muffins or cookies. With cakes most innovation is in flavoring, frostings, and decorations. Muffins (and quick breads) let you experiment with different grains, fillings like raisins and nuts, and purees (banana, pumpkin, etc).
In addition to the bakers percentages, it's important to understand your ingredients so you have a good understanding how to swap out fats, or flours, or sugars to achieve the texture or flavor you are looking for. You can start with a recipe you like and know is successful, then switch out ingredients to attempt specific results.
It helps to think not about recipes in baking but about formulas and techniques. Formulas are organized around the percentage of flour in a formula and is called 'Baker's Percentage' . The techniques you mostly already know: ie the muffin method, the creaming method.
As an exercise: re write a favorite recipe into bakers percentage and I think you will start to see them differently in a way that will free up your creativity.
Here is a like that explains Baker's Percentage better than I could:
It also might be a good idea to consider getting a Baking/Pastry textbook. There are some good ones out there.
Good luck, and I hope I didn't make this sound harder than it needs to be!
While I don't bake, I do create my own recipes for other things. After having my husband ask, "Hey, can you make this again?" repeatedly and the answer always being no, I've learned to keep some scratch paper and a pen in the kitchen with me while I play. I scribble things quickly, sometimes even with my non dominant hand, while I work. Once I've tasted the finished product, I clean up and start typing my notes in a much cleaner format in Evernote.