The following is taken directly from http://nasomi.com/instruction.php
Please follow these instructions as closely as possible. It's very easy to miss a step or overlook something, and then nothing will work.
This is a personal guide I've put together as some folks may not be particularity computer savvy, or may not speak English as their native language. If you have any tips, comments, images for a different OS to share about this guide please email me email@example.com. If you are having trouble, go to IRC and ask for help with your installation. Please don't email me, I don't check this email enough to get a reply back to you.
* uTorrent is needed in order to download the files you will need to continue.
Step 1: Download the Final Fantasy XI Installer as a torrent here.
Step 2. 32-bit users launch FFXISETUP.exe and install everything.64-bit Windows users launch 64bit POL, then launch FFXISETUP.exe and install everything EXCEPT Playonline
Notice: I am using Windows 8 to complete this install, so you may have a few different details or the images may not exactly match your screen. - Petz
64-bit Windows users launch 64bit POL, then launch FFXISETUP.exe and install everything EXCEPT PlayOnline
Step 2. 32-bit users launch FFXISETUP.exe and install everything.
Core FFXI is now installed! Just the rest of the expansions to go… Going to be short and to the point on these. -Petz
Step 3: Open the “update files” folder, and select all the files and copy them
Step 4: Open the directory you installed into, for example: C:\Program Files\PlayOnline\SquareEnix\Final Fantasy XI\
Step 5: Paste the update files you just copied into this folder. Click overwrite on any duplicates.
Step 6: Launch PlayOnline, it will briefly update, and then restart.
Step 7: Click “Existing Members” and enter POL ID ABCD1234 and click register, then click login
Step 8: On the left side click “Check Files” and CHANGE THE DROPDOWN to read FINAL FANTASY XI.
Step 9: Click “Check Now”, this process will take some time. Eventually it will come up with “Repair”. Select that. This can take a few hours.
Step 10: Open your install directory and find Playonline\SquareEnix\PlayonlineViewer\ to find and copy the 'data' folder.
Step 11: Back out and open Playonline\SquareEnix\FINAL FANTASY XI\ and paste the data folder into this directory.
Step 12: Open the “Visual C++ Runtimes” folder and install ALL 3 files in the folder.You will now ALSO need to CLICK HERE AND DOWNLOAD an additional runtime, and install it as well.
The install page has the link, so if you need it go to http://nasomi.com/instruction.php and locate the link in step 11 of the page.
Step 13: Open the “Ashita” folder and right click Ashita and run as administrator.
The Ashita folder is already in the location that it needs to be if you used the Torrent to download. If not, locate the folder that we've been calling the default location and move the folder there. Do not try and run the game with the Ashita folder somewhere else!
Step 14: Click on one of the server profiles and edit to suit your preferences, or double click to launch.
Step 15: Follow the instructions on the black screen.