First of all, you need the file, it's normally a .82, but for updated ET versions you'll get .84 demos.
An example of these demos can be found here: Here
So first of all you gotto figure out in what MOD it is recorded.
If it's a Noquarter demo, A jaymod demo or something else. Because that's important.
Example on this one I gave as an example is NQ.
first step: You put the file in your modfolder. This can be found on the hard drive you've installed your ET on. Example for me is: C:\Windows.old\Program Files\Wolfenstein - Enemy Territory\noquarter\demos
You put the file in the Demos folder.
After that you open up your ET. When you are at main menu in ET, go to MODS

You select the MOD you need (in our example: Noquarter) and ET will restart with the selected MOD

After you've selected the right mod, click on the button: Replays (or demos)

You'll see a list of demos that are in the mod folder (example: Noquarter)
These demos may have the following name: 2009-09-02-165247-Caen. That means they are recorde on the 2nd of septembre 2009 in the map Caen.
Chose the selected demo.
you'll be loading the demo.
Once opened the demo, you'll be having a control pannel that looks like this, it's useful!

Anyways, hope this helps, if not, ask questions, i'll answer them for you!
David




















