How do I substitute a recipe that calls for yellow cornmeal for self rising corn meal mix
It sounds like you're just making a cornbread (more of a sodabread with cornmeal in it) that has some "fun" things added to it. If that's the case, find whatever cornbread recipe is on the side of your self-rising cornmeal-and-flour container and add cheddar, jalapeno, and scallions.

Since we do not know the proportions that constitute the mixture, we have to guess. This recipe suggests that a traditional cornbread is approximately a 1:1:1 ratio (1c corn meal, 1c flour, 1 tbsp baking powder). I'll assume these are the proportions of your mixture.

I would take 2 cups (and a tablespoon) of self-rising cornmeal mixture, then add 2 cups of flour, and 1 tbsp of baking powder: in total that would make 1 cup of corn meal, 3 cups of flour, and 2 tablespoons of leavening cornmeal. The rest can be figured from there.

Then never buy self-rising anything again! It's much easier and more versatile to just stock baking soda and baking powder as pantry essentials.