Agreed and extended:
You can:
- Sculpt the plates separately.
- Reshape the edge several times while it hardens. Pushing it against a sharp corner may help.
- Let it harden and then cut it with a very sharp blade. Dull ones will just tear it.
Sanding/filing GS is no fun. It's possible, but takes good tools and lots of time. Avoid it if you only can. With GS, all time is better spent making the original sculpt perfect than trying to correct it afterwards.
Alternatively, you can use other putties. Brown stuff and Procreate are a bit better, but still somewhat rubbery. That is, they may tear. Consider getting some clay-like putty like Magic Sculp or Milliput. They can be filed, drilled and sanded easily.
Or: if possible, use ready made parts, plastic profiles, tin bits from sprues etc.
Green stuff is fine for general work, but it's also among the softest putties out there. Like someone said, it's like trying to shape chewing gum sharply. Other tools may be a better idea than using GS stubbornly for purposes where it doesn't excel.