Ok, first, you are going to need Tortoise SVN. To download it, go here: http://tortoisesvn.net/downloads. When you are done downloading that, it will ask you to restart your computer, you can choose yes or no, it's your decision.

Ok, next you will need to create a folder in your Garry's Mod Addons folder, called phx3. Then, right click that folder, and choose SVN Checkout. When the dialog box opens up for the SVN Checkout, in the repository section, put in:

https://phoenix-storms.svn.sourcefor...hoenix-storms/

What is above, is the PHX3 SVN Repository. Then you just let it download, and whenever you need to update, right-click your PHX3 folder and select SVN Update.