The demo packages are available as a standard Zip file, and as a Dzip file. Dzip is a compression utility which compresses Quake demos better than Zip does. You can get it here if you do not already have it.

Once you've made your choice and downloaded, installation and running is straightforward: You should make a subdirectory of your main Quake directory called, e.g. qdqwav. This is where you should unzip all the contents of your download to.

When you have done this, you can start QdQwav by running Quake with the -game qdqwav command line option. If you need more precise instructions, you should refer to the text file.