How do you make xanthan gum?

No. The two ingredients have entirely different functions.

Baking powder contains an acid and baking soda, so that when the mixture is wetted the acid and soda react to form carbon dioxide bubbles which in turn causes the cake to rise.

Xanthan gum, on the other hand, is a thickener, stabiliser or gelling agent which is stable at a wide range of temperatures, but is more viscous when cooler.

If you used baking powder together with xanthan gum in the recipe, you could expect a very light and fluffy cake to come out of the oven, which would probably look more like a pancake if you chose to use neither.