How to write frames to a video file?

This might help get you started - the documentation is available, but newer features tend to be documented in ffmpeg's man pages.

The frames need to be numbered sequentially.

ffmpeg -f image2 -framerate 25 -i frame_%d.jpg -c:v libx264 -crf 22 video.mp4

• -f defines the format
• -framerate defines the frame rate
• -i defines the input file/s ... %d specifies numbered files .. add 0's to specify padding, e.g. %05d for zero-padded five-digit numbers.
• -vcodec selects the video codec
• -crf specifies a rate control method, used to define how the x264 stream is encoded
• video.mp4 is the output file

For more info, see the Slideshow guide.