Help a nub: How to (properly) install addons?

Everyone has that time during which they suck at something. This is my time in which I suck at Garry’s Mod and all related activities. All of them.

I couldn’t find good answers anywhere else, so I decided to ask it here: How do I properly install and use addons for Garry’s Mod? Is it different depending on the contents of the download? Why does it have that purple stuff on it when I spawn it?

I’m asking because I recently found out that it’s not as simple as just dropping it into the addon folder.

Thanks and sorry if I just caused you all to facepalm.

I wouldnt “facepalm”, as I have seen alot of people have this problem.

First, download the addon, then check the folder for a file called “info.txt”.
If info.txt does exist, place the folder it is in to
C:/program files/steam/steam apps/<your steam name>/garrysmod/garrysmod/addons/
If the file does not have a text file called info.txt, read the readme.
If the file does not have a readme, contact the author for further information.
If the author does not answer, refer to asking from me by PM.

Some addons use SVN. Here’s an SVN tutorial:
For those that don’t, read the post above mine.

Ah, yes. I almost forgot SVN, thank you Divran.

Thanks Hakita.

No problem, Im glad I helped someone.