I should write a tutorial on that.
basically, there are two methods. 1. while playing back the demo, type
in console "startmovie <name> <fps>" ... this will take
screenshots from the demo, and put them as bmps in your half-life (or
whatever mod's folder) at whatever fps you want. as in, if you chose 30
fps then 10 seconds would be 300 bmps. quite a lot. anyway after that
you get a program like Videomach to put all the frames together into an
avi or mpg or whatever ... then you'd use a video editing program like
premier or vegas or after effects to put the different avis or "clips"
together with effects, music, etc ... into a movie.
the 2nd way is to use a program such as fraps -
www.fraps.com - to
directly make avis (with sound!) from your demo... you run fraps in the
background, then play your demo back, and press F2 or whatever your
button is to start recording, and fraps turns everything you see on
screen into an uncompressed avi in a folder of your choosing.
for example, fraps was used to make the hl in under an hour movies