any advice for dying the underlayer?

Yep, I just did mine black. It was a bit tricky  since you can't really see the back of your head. Measure how much you want red, maybe you just want a skiff or practically the whole under layer. Pin the top hair up really high and as close to your head as possible so it won't fall into the color. And then knock yourself out.  Apply the color and then rinse out. If you have blonde hair be very carefully the color will RUN into the blonde and tint it a bit. I had that trouble with the black, I'm waiting for it to fade. If you have dark hair then you should be fine.