About the program
Full information about the NSF GRFP can be found here. I applied for it at the same time as I applied to neuroscience PhD programs. It's a really great opportunity- they take about 2000 fellows each year spanning the sciences (excluding clinically focused work).
Applications are usually due in late October, so I recommend starting them over the summer. Feel free to reach out to me if you have questions! Good luck.
Alex Lang’s website has lots of examples and great advice: https://www.alexhunterlang.com/nsf-fellowship
My application materials
In case they are helpful for your application, here are my submitted materials, minus my transcript:
Personal Statement
Research Proposal
Feedback
CV