First off go to garrysmod.org and find the mod/map/addon you want. If you don't have anything to extract rar files or 7z files please consider this as your best option its free and open source: 7-Zip.
1. Download the file or files you want for your gmod.
2. Open them up with 7-Zip or what ever program you use to open compressed files.
3. Extract the file or files to your C:\Program Files\Steam\steamapps\*Your Steam Username*\garrysmod\garrysmod folder.
Now go back to the part where you had the compressed file open in 7-Zip or what ever program that can open compressed files and now extract it to C:\Program Files\Steam\steamapps\*Your Steam Username*\garrysmod\garrysmod\maps folder.
Repeat the same action you did with Map Installation but this time extract it to C:\Program Files\Steam\steamapps\*Your Steam Username*\garrysmod\garrysmod\addons folder instead.
Hopefully This is what some people are looking for because the other one tutorial made by deacal wasn't very good. If you want to suggest more installation help in this tutorial please leave a comment with your suggestion.