1. Import your video footage into After Effects.
2. Create a new composition and drag your footage onto the timeline.
3. Right-click on your footage layer and select "Interpret Footage > Main."
4. In the "Time Remapping" section, change the "Frame Rate" to a lower value. This will slow down your footage and create the long exposure effect.
5. To add motion blur to your footage, open the "Effect Controls" panel and navigate to the "Blur & Sharpen" category. Apply the "Motion Blur" effect to your footage layer.
6. Adjust the "Shutter Angle" value to your desired amount. Higher values will create more motion blur.
7. To further exaggerate the long exposure effect, you can add a grain effect to your footage. Open the "Effect Controls" panel and navigate to the "Noise & Grain" category. Apply the "Grain" effect to your footage layer.
8. Adjust the "Intensity" and "Size" values to your desired amount. Higher values will create more grain.
9. Finally, adjust the timing of your long exposure effect. Use the "Time Remapping" controls on your footage layer to extend the desired section of the video.