http://www.skylabsonline.com/blog/project/octgn-w-lobby/

Instructions are pretty straight-forward.

If the links don't work, try this one instead: http://www.skylabsonline.com/octgndownload/publish.htm

Download:
First, Download OCTGN from this location:
http://www.octgn.net/downloads.php

Next, download the Game Definition File and Markers file from this location:
http://octgnsets.freeforums.org/mtg-game-definition-version-2-0-7-updated-april-25-2011-t3.html

Then, download all sets from here (Except for those catergorized as "Promo Cards"):
http://octgnsets.freeforums.org/mtg-set-definition-files-updated-june-14-2011-t4.html

Install:
SPECIAL NOTE - THE PROGRAM RELIES ON THE LOCATION OF WHERE YOU HAVE THE SETS; DO NOT MOVE THE SETS OR DEFINITION FILES AFTER INSTALLING THEM.

Run the OCTGN program for the first time. The program should request to patch. Allow it to do so.

Once the Main Menu is on-screen, click on the "Manage installed games" button on the right. From there, click "Install a new game". Navigate to where you have stored the MTG game definition file, and "Open" that.

Afterward, click "Install new cards" to install all the sets you downloaded as well as the Markers file in the same fashion you installed the Game Definition File.

Congratulations! You are now ready to build some decks! Note that some sets install some pre-constructed decks. Use, move, or delete these at whim.