record a demo with /record and /stoprecord in the console. it should print the file name, which it has automatically chosen for the demo, in the console
tell quake to automatically record screenshots at a given framerate /setÊcl_avidemoÊ<framerate>
play your demo, /demoÊdemo0001 (where demo0001 is the file name quake used)
turn off screenshot recording /setÊcl_avidemoÊ0
compile the screenshots in HOME/.q3a/baseq3/screenshots/ into a movie
notes
If you are running a mod (eg, plot), the screenshots will appear in HOME/.q3a/plot/screenshots/. Similarly, if you are on OsX, instead of HOME/.q3a, look for something like /Users/user/ApplicationÊSupport/Quake3.