You can find ffmpeg builds for Windows here: https://ffmpeg.zeranoe.com/builds/ (use static linking)
Extract the downloaded zip file into your .minecraft
folder.
You should get a file named ffmpeg-20170801-1193301-win64-static
(or similar), rename that file to just ffmpeg
.
You've done everything correctly if you end up with the ffmpeg executable at .minecraft/ffmpeg/bin/ffmpeg.exe
.