1. Select File > Import > Video Frames to Layers
2. Trim down to 500 frames
3. Adjust import settings and click OK - Use "Limit to Every 2 Frames" to decrease size - Keep "Make Frame Animation" selected
4. Edit in Photoshop as usual using Adjustment layers, Image Size, Text, etc.
5. Edit invidual layers if needed
6. Save as animated GIF using File > Save for Web