1. Insert the Warcraft 2 CD
The hard part is to obtain the original game CD. This mod is not even really a Warcraft 2 mod - it's a Wargus mod. Wargus is the improved version of WC2 made by enthusiasts. To ensure Blizzard is not going to send assassins to hunt them down, the creators of Wargus made their game require the Warcraft 2 CD or image to install. Since your first step is to install Wargus, you'll need a CD or image file of the CD of either Warcraft 2 Tides of Darkness, or Warcraft 2: Battle.net Edition. I only had experience using the latter.
So, how do you get the "Warcraft 2: Battle.Net Edition" image? I hope not via torrents, which is easy, but illegal. Take the legal path instead - respect Blizzard's property rights for this 1995 game.
If you have the image*, which is possibly an .iso file, you need to mount it For that I use DAEMON Tools Lite. Once you have both the image and the program installed, you'll be able to mount the .iso file, by either clicking or right-clicking on it (you'll need to choose a proper option from the menu).
You're now supposed to have the CD mounted. In my case - to drive K:\
* obtained by backing up your original game CD, not via torrents
2. Install Wargus, get the wargus.exe file
Download the mod - version V4C1. Run the Wargus-2.4.exe file found inside.
First time you're asked, enter the path of the mounted image. Like "K:\". The second time you're asked, just leave the field blank. You're now supposed to have Wargus installed to somewhere, like C:\Wargus. Go there and grab wargus.exe
3. Unpack the mod
Unpack the "Wargus + Nightmoon V4c" folder anywhere. Put wargus.exe inside, run it. If something, right-click and "run as Administrator". That's all there is.
4. Patches & Maps
The relevant patch currently is V4C3. It contains the "scripts" folder, so you need to replace the original "scripts" folder from where you installed the game with this one.
To install maps, put the individual files strictly to the maps\ folder.