Installing Armoured Combat Framework is pretty simple but has a couple of extra steps on top of your usual installation of an SVN add-on but nothing a little bit of copy-pasting can't handle. Instructions below.
First things first, you need to have TortoiseSVN or similar installed so you can check out the subversion (SVN) repository for ACF. SVN is a popular way for GMod add-on developers to distribute projects they are actively developing so you'll probably use it for other add-ons as well. You can get a copy of TortoiseSVN Here.
Next make a folder in your Garry's Mod addons directory
C:\Program Files (x86)\steam\SteamApps\common\GarrysMod\garrysmod\addons
called ACF. Once done right-click on your ACF folder and click SVN checkout. A new window will appear with a field where you can paste in the URL https://github.com/nrlulz/ACF/trunk
and then hit OK.
If everything is correct TortoiseSVN will download the ACF files into your ACF directory.
Once complete you need to copy the scripts and particles directories from your ACF directory into your main Garry's Mod directory. If you don't it just won't work. You'll get asked to overwrite a few things, do it, and you're done.
C:\Program Files (x86)\steam\SteamApps\common\GarrysMod\garrysmod\particles
This installation method will work for both single-player and server installations. There is no longer a server-specific install.