Signature requirements - confused about format

Getting frustrated here - my photo has been rejected twice now and I'm not sure what I'm doing wrong.

First rejection: "Lighting too dark"
Second rejection: "Background not plain enough"

I'm using a white wall as background and taking the photo during the day with window light. What else should I be doing?

Can someone who's had their photo accepted on first try share exactly how they took it? Camera settings, distance, lighting setup, everything. I really want to get this right.