After installing yt-dlp and ffmpeg you can run the following command to download a youtube video and convert it to an instagram reel.

yt-dlp -o ski.webm https://www.youtube.com/watch?v=XKnz65lTHPM&ab_channel=RedBullSnow

and then

ffmpeg -i ski.webm -ss 00:01:25 -to 00:02:30 -filter_complex "[0:v]crop=1080:1920:(iw-1080)/2:(ih-1920)/2[v]" -map "[v]" -map "0:a?" -c:v libx264 -c:a copy output.mp4

The first command, yt-dlp, is a modern and updated version of youtube-dl. You suppodely can do features like grab specific timestamps of the a youtube video, but I also read it can take longer than just downloading the whole video. I tried to use youtube-dl but it was downloading videos painfully slow.

The second command is your typical ffmpeg command. We are taking an input:

-i ski.webm

and then we are specifying a start time:

-ss 00:01:25

and an end time:

-to 00:02:30

We are then cropping the video to be 1080x1920:

-filter_complex "[0:v]crop=1080:1920:(iw-1080)/2:(ih-1920)/2[v]"

. We are then mapping the video and audio:

-map "[v]" -map "0:a?"

We are then specifying the video codec:

-c:v libx264

and the audio codec:

-c:a copy

And finally we are specifying the output file:

output.mp4