FallenWyvern
Villager
If your computer says something like "Unknown file extension RAR", you'll want to install 7-zip to be able to unzip the DND_monster file. Get the 7-zip MSI here: http://www.7-zip.org/
Then you can right-click on the DND_Monster RAR file in the file explorer and tell it to "extract here". It will create a folder called DND_Monster, and within that folder will be a program called DND_Monster.exe. Double-click on that program and it will start.
Using 7-zip to extract the folder is what FallenWyvern means by "decompress it anywhere."
Good point. I totally went under the assumption that people know what compressed folders are. My bad. I should make an installer for this to save people some trouble.