Mix approximately equal parts of the baking soda and bleach together in a bowl. You should end up with a thick paste that is easy to spread. Add extra bleach if it is too thick and more baking soda if it is too runny.
After thoroughly mixing the paste together, apply it to any dirty surfaces in the shower. There should be just enough to cover the surface of the mold. It is best if this is down at the end of the day when nobody is expecting to use the shower. Also, bleach gives off a lot of chemical vapors so be sure to keep the fan running or open a window.