Start with mostly corn starch and add in pinches of cocoa powder at a time. For my complexion, I need very little cocoa powder and have to be careful not to add to much. Once you find the right mixture it gets easier and easier each time you mix it! Depending on your shade, you can also add in beet root powder or make an easy blush with corn starch and beet root powder alone!
If you don't have corn starch or want an alternative, arrowroot powder is also a great base to mix with and won't change the color too much.
I keep it in a wide container that I can fit a loose brush inside for easily application. Shake the container every once in a while as moisture may cause the powder to start to clump and separate. It is easily fixed with a shake or two but has definitely left me with some brown spots when the cocoa powder gets too friendly and sticks together!
This powder does not act as a foundation and won't really stick to your skin without some liquid foundation or moisturizer under it. I use it strictly as a setting powder to apply after my liquid foundation and to smooth my complexion. If you want to use this as a foundation alone, you can try mixing it with a facial lotion or something such as shea butter to create your shade. I haven't quite mastered that yet but will be sure to share when I do!
Enjoy smelling lightly of chocolate! :)