ffmpeg -f x11grab -video_size 1366x768 -i $DISPLAY -f alsa -i default -c:v libx264 video.mp4

Adjust the video size to your screen resolution for best quality.

Cutting the video

Use Pitivi, OpenShot or another GUI program.

Command line

Can be faster, as it does not need to re-encode

Cutting

ffmpeg -ss 00:08:00 -i video.mp4 -ss 00:01:00 -t 00:01:00 -c copy videocut.mp4

The first -ss seeks fast to (approximately) 8min0sec, and then the second -ss seeks accurately to 9min0sec, and the -t 00:01:00 takes out a 1min0sec clip.

https://superuser.com/questions/138331/using-ffmpeg-to-cut-up-video

Use -to instead of -t to seek to absolute time values.

ffmpeg -ss 00:08:00 -i video.mp4 -ss 00:01:00 -to 00:10:00 -c copy videocut.mp4

Joining videos

https://trac.ffmpeg.org/wiki/Concatenate