The Manual and easiest way

If you don't have the option or want't to install git on your system, then go to

https://github.com/Pymol-Scripts/Pymol-script-repo

Click the "ZIP" button, near the top left corner. Now unpack the files to a directory. For example:

C:/Users/YOURNAME/Documents/Pymol-script-repo

The Git way - For windows users

Use following settings in options, (You can read more here).

Windows Explorer Integration -> Context Menu Entries -> Gut Bash Here + Git GUI here
Run Git and included Unix tools from Windows Command prompts
Checkout Windows style, commit Unix-style endings

Navigate to: C:/Users/YOURNAME/Documents (Note, always use forward slashes "/" for compatibility)
Right click in folder -> Select: Git Gui -> Clone Existing Repository
Source Location: git://github.com/Pymol-Scripts/Pymol-script-repo.git
Target Directory: C:/Users/YOURNAME/Documents/Pymol-script-repo

You now have all the scripts available in your directory.

You now have to add the "Pymol-script-repo" directory to the PyMOL search path.

import sys
sys.path.append('C:/Users/YOURNAME/Documents/Pymol-script-repo')

Save under: C:/Users/YOURNAME/pymolrc.pym (Set: "Save as type" to "All files")

Now start PyMOL, and test in PyMOL.

print sys.path
import colorbydisplacement
help(colorbydisplacement)

If new scripts are available or changes have been made, then:

git pull origin master

sudo apt-get install git

git clone git://github.com/Pymol-Scripts/Pymol-script-repo.git

You now have all the scripts available in: /home/YOURNAME/Software/pymol/Pymol-script-repo

You now have to add the "Pymol-script-repo" directory to the PyMOL search path.

gedit ~/.pymolrc

Write

import sys
sys.path.append('/home/YOURNAME/Software/pymol/Pymol-script-repo')

Save and exit

Now start PyMOL, and test in PyMOL.

print sys.path
import colorbydisplacement
help(colorbydisplacement)

If new scripts are available or changes have been made, then:

git pull origin master

Information how to contribute scripts to the repository. It's easy!
http://www.pymolwiki.org/index.php/git_authors