I’ve been doing something I assume it’s similar, my drawing skills are way worse though. I put a piece of cardboard or wood inside the shirt, clamp it with something to make it easier to draw without the shirt moving around. I pour some bleach in a shot glass and use a paintbrush. When you’re done soak the bleached area with peroxide to stop it from continuing to react. Experiment on some expendable material first. Also I use some chalk to roughly get an idea of are and scale I’m drawing.