For the GRE, blindly follow everything my friend Prottay (167Q 170V) describes here. My 2 cents would be to start in advance. The earliest deadlines are towards the end of November/early December. If you are working alongside preparing for the GRE, I would suggest at least 3-4 months of prep time. If you are not working, 2 months of focused work should do.  The GRE is not an exam of intelligence, hence enough practice will help you. For Economics/PubPol, they will only care about your Quant score. Focus on getting at least 165. If you are aiming for the stars, 168+ is your only option.

Aim to sit for the GRE around August. This leaves 3ish months in case you need to retake it. Once the GRE is out of the way, you can use the remaining time to focus on writing the application materials.